ATI Physical Therapy (ATIP) 10-K27 Feb 242023 FY Annual reportFinancial data

Company Profile

Cover

Cover - USD ($) $ in Millions	12 Months Ended
Cover - USD ($) $ in Millions	Dec. 31, 2023	Feb. 19, 2024	Jun. 30, 2023
Document Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2023
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-39439
Entity Registrant Name	ATI Physical Therapy, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	85-1408039
Entity Address, Address Line One	790 Remington Boulevard
Entity Address, City or Town	Bolingbrook
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60440
City Area Code	630
Local Phone Number	296-2223
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction	false
Entity Shell Company	false
Entity Public Float			$ 17
Entity Common Stock, Shares Outstanding		4,210,286
Amendment Flag	false
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
Entity Central Index Key	0001815849
Common Class A
Document Information [Line Items]
Title of 12(b) Security	Class A common stock, $0.0001 par value
Trading Symbol	ATIP
Security Exchange Name	NYSE
Redeemable Warrants, exercisable for Class A common stock at an exercise price of $575.00 per share
Document Information [Line Items]
Title of 12(b) Security	Redeemable Warrants, exercisable for Class A common stock at an exercise price of $575.00 per share
Trading Symbol	ATIPW

Audit Information

Audit Information	12 Months Ended
Audit Information	Dec. 31, 2023
Auditor Information [Abstract]
Auditor Name	Deloitte and Touche LLP
Auditor Location	Chicago, IL
Auditor Firm ID	34

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands		Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents		$ 36,802	$ 83,139
Accounts receivable (net of allowance for doubtful accounts of $48,055 and $47,620 at December 31, 2023 and December 31, 2022, respectively)		88,512	80,673
Prepaid expenses		12,920	13,526
Insurance recovery receivable		23,981	933
Other current assets		4,367	9,107
Assets held for sale		2,056	6,755
Total current assets		168,638	194,133
Property and equipment, net		100,422	123,690
Operating lease right-of-use assets		194,423	226,092
Goodwill, net		289,650	286,458
Trade name and other intangible assets, net		245,858	246,582
Other non-current assets		4,290	2,030
Total assets		1,003,281	1,078,985
Current liabilities:
Accounts payable		14,704	12,559
Accrued expenses and other liabilities		88,435	53,672
Current portion of operating lease liabilities		51,530	47,676
Liabilities held for sale		1,778	2,614
Total current liabilities		156,447	116,521
Long-term debt, net	[1]	433,578	531,600
2L Notes due to related parties, at fair value		79,472	0
Warrant liability		3	98
Contingent common shares liability		578	2,835
Deferred income tax liabilities		21,367	18,886
Operating lease liabilities		185,602	218,424
Other non-current liabilities		1,696	1,834
Total liabilities		878,743	890,198
Commitments and contingencies (Note 17)
Mezzanine equity:
Series A Senior Preferred Stock, $0.0001 par value; 1.0 million shares authorized; 0.2 million shares issued and outstanding; $1,249.06 stated value per share at December 31, 2023; $1,108.34 stated value per share at December 31, 2022		220,393	140,340
Stockholders' equity:
Class A common stock, $0.0001 par value; 470.0 million shares authorized; 4.2 million shares issued, 4.0 million shares outstanding at December 31, 2023; 4.1 million shares issued, 4.0 million shares outstanding at December 31, 2022		0	0
Treasury stock, at cost, 0.007 million shares and 0.002 million shares at December 31, 2023 and December 31, 2022, respectively		(219)	(146)
Additional paid-in capital		1,308,119	1,378,716
Accumulated other comprehensive income		406	4,899
Accumulated deficit		(1,409,306)	(1,339,511)
Total ATI Physical Therapy, Inc. equity		(101,000)	43,958
Non-controlling interests		5,145	4,489
Total stockholders' equity		(95,855)	48,447
Total liabilities, mezzanine equity and stockholders' equity		$ 1,003,281	$ 1,078,985

[1] Includes $17.0 million of principal amount of debt due to related parties as of December 31, 2023.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands		Dec. 31, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Allowance for doubtful accounts		$ 48,055	$ 47,620
Preferred stock, par value (in dollars per share)		$ 0.0001	$ 0.0001
Temporary Equity, Preferred stock, shares authorized (in shares)		1,000,000	1,000,000
Temporary Equity, Preferred stock, shares issued (in shares)		200,000	200,000
Temporary Equity, Preferred stock, shares outstanding (in shares)		200,000	200,000
Preferred stock, stated value (in dollars per share)		$ 1,249.06	$ 1,108.34
Common stock, par value (in dollars per share)		$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)		470,000,000	470,000,000
Common stock, shares Issued (in shares)		4,200,000	4,100,000
Common stock, shares outstanding (in shares)		4,000,000	4,000,000
Treasury stock (in shares)		6,794	2,000
Long-term debt, net	[1]	$ 433,578	$ 531,600
Related Party
Long-term debt, net		$ 17,000

[1] Includes $17.0 million of principal amount of debt due to related parties as of December 31, 2023.

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Net revenue	$ 699,016	$ 635,671
Cost of services:
Salaries and related costs	382,370	357,982
Rent, clinic supplies, contract labor and other	208,593	202,568
Provision for doubtful accounts	11,251	13,869
Total cost of services	602,214	574,419
Selling, general and administrative expenses	118,728	114,724
Goodwill, intangible and other asset impairment charges	5,591	486,262
Operating loss	(27,517)	(539,734)
Change in fair value of 2L Notes	(24,471)	0
Change in fair value of warrant liability	(95)	(4,243)
Change in fair value of contingent common shares liability	(2,257)	(42,525)
Interest expense, net	61,039	45,278
Other expense, net	1,777	3,333
Loss before taxes	(63,510)	(541,577)
Income tax expense (benefit)	2,568	(48,530)
Net loss	(66,078)	(493,047)
Net income (loss) attributable to non-controlling interests	3,717	(668)
Net loss attributable to ATI Physical Therapy, Inc.	(69,795)	(492,379)
Less: Series A Senior Preferred Stock redemption value adjustments	38,958	0
Less: Series A Senior Preferred Stock cumulative dividend	23,219	17,876
Net loss available to common stockholders, diluted	(131,972)	(510,255)
Net loss available to common stockholders, basic	$ (131,972)	$ (510,255)
Loss per share of Class A common stock:
Basic (in dollars per share)	$ (31.93)	$ (125.59)
Diluted (in dollars per share)	$ (31.93)	$ (125.59)
Weighted average shares outstanding:
Basic (in shares)	4,133	4,063
Diluted (in shares)	4,133	4,063
Net patient revenue
Net revenue	$ 636,095	$ 575,940
Other revenue
Net revenue	$ 62,921	$ 59,731

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Statement of Comprehensive Income [Abstract]
Net loss	$ (66,078)	$ (493,047)
Other comprehensive (loss) income:
Cash flow hedges	(4,493)	4,871
Comprehensive loss	(70,571)	(488,176)
Net income (loss) attributable to non-controlling interests	3,717	(668)
Comprehensive loss attributable to ATI Physical Therapy, Inc.	$ (74,288)	$ (487,508)

Consolidated Statements of Chan

Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Treasury Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Non-Controlling Interests
Beginning balance (in shares) at Dec. 31, 2021		3,948,199
Beginning balance at Dec. 31, 2021	$ 511,507	$ 0	$ (95)	$ 1,351,617	$ 28	$ (847,132)	$ 7,089
Beginning balance (in shares) at Dec. 31, 2021			596
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of 2022 Warrants	19,725			19,725
Vesting of restricted shares distributed to holders of ICUs (in shares)		7,207
Issuance of common stock upon vesting of restricted stock units and awards (in shares)		12,684
Tax withholdings related to net share settlement of restricted stock awards (in shares)		(944)	944
Tax withholdings related to net share settlement of restricted stock units and awards	(51)		$ (51)
Non-cash share-based compensation	7,374			7,374
Other comprehensive income (loss)	4,871				4,871
Distribution to non-controlling interest holders	(1,932)						(1,932)
Net (loss) income attributable to non-controlling interests	(668)						(668)
Net loss attributable to ATI Physical Therapy, Inc.	$ (492,379)					(492,379)
Ending balance (in shares) at Dec. 31, 2022	4,000,000	3,967,146
Ending balance at Dec. 31, 2022	$ 48,447	$ 0	$ (146)	1,378,716	4,899	(1,339,511)	4,489
Ending balance (in shares) at Dec. 31, 2022	2,000		1,540
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Series A Senior Preferred Stock dividends and redemption value adjustments	$ (80,053)			(80,053)
Capital contribution from recognition of delayed draw right asset	$ 690			690
Vesting of restricted shares distributed to holders of ICUs (in shares)		2,873
Issuance of common stock upon vesting of restricted stock units and awards (in shares)		41,510
Tax withholdings related to net share settlement of restricted stock awards (in shares)	5,254	(5,254)	5,254
Tax withholdings related to net share settlement of restricted stock units and awards	$ (73)		$ (73)
Issuance of common stock for fractional adjustments related to Reverse Stock Split (in shares)		26,346
Non-cash share-based compensation	8,766			8,766
Other comprehensive income (loss)	(4,493)				(4,493)
Distribution to non-controlling interest holders	(3,061)						(3,061)
Net (loss) income attributable to non-controlling interests	3,717						3,717
Net loss attributable to ATI Physical Therapy, Inc.	$ (69,795)					(69,795)
Ending balance (in shares) at Dec. 31, 2023	4,000,000	4,032,621
Ending balance at Dec. 31, 2023	$ (95,855)	$ 0	$ (219)	$ 1,308,119	$ 406	$ (1,409,306)	$ 5,145
Ending balance (in shares) at Dec. 31, 2023	6,794		6,794

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022
Operating activities:
Net loss		$ (66,078)	$ (493,047)
Adjustments to reconcile net loss to net cash used in operating activities:
Goodwill, intangible and other asset impairment charges		5,591	486,262
Depreciation and amortization		37,412	40,590
Provision for doubtful accounts		11,251	13,869
Deferred income tax provision		2,481	(48,573)
Non-cash lease expense related to right-of-use assets		47,926	48,253
Non-cash share-based compensation		8,766	7,374
Amortization of debt issuance costs and original issue discount		2,889	2,873
Non-cash interest expense		6,567	3,481
Loss on extinguishment of debt		444	2,809
Loss on disposal and sale of assets		1,743	9
Change in fair value of 2L Notes		(24,471)	0
Change in fair value of warrant liability		(95)	(4,243)
Change in fair value of contingent common shares liability		(2,257)	(42,525)
Change in fair value of non-designated derivative instrument		475	0
Changes in:
Accounts receivable, net		(18,604)	(12,573)
Insurance recovery receivable		(23,048)	7
Prepaid expenses and other current assets		3,595	(5,031)
Other non-current assets		(2,413)	39
Accounts payable		1,138	(48)
Accrued expenses and other liabilities		42,017	854
Operating lease liabilities		(47,732)	(53,628)
Other non-current liabilities		37	28
Medicare Accelerated and Advance Payment Program Funds		0	(12,288)
Net cash used in operating activities		(12,366)	(65,508)
Investing activities:
Purchases of property and equipment		(17,322)	(28,147)
Proceeds from sale of property and equipment		91	157
Proceeds from sale of clinics		355	77
Payment of holdback liabilities related to acquisitions		(490)	(135)
Net cash used in investing activities		(17,366)	(28,048)
Financing activities:
Proceeds from long-term debt		0	500,000
Proceeds from 2L Notes from related parties		3,243	0
Financing transaction costs		(6,287)	0
Deferred financing costs		(84)	(12,952)
Original issue discount		0	(10,000)
Principal payments on long-term debt		0	(555,048)
Proceeds from issuance of Series A Senior Preferred Stock		0	144,667
Proceeds from issuance of 2022 Warrants		0	20,333
Proceeds from revolving line of credit		35,000	48,200
Payments on revolving line of credit		(44,750)	0
Equity issuance costs and original issue discount		0	(4,935)
Payment of contingent consideration liabilities		(593)	(203)
Taxes paid on behalf of employees for shares withheld		(73)	(51)
Distribution to non-controlling interest holders		(3,061)	(1,932)
Net cash (used in) provided by financing activities		(16,605)	128,079
Changes in cash and cash equivalents:
Net decrease in cash and cash equivalents		(46,337)	34,523
Cash and cash equivalents at beginning of period		83,139	48,616
Cash and cash equivalents at end of period		36,802	83,139
Supplemental noncash disclosures:
Derivative changes in fair value	[1]	4,493	(4,871)
Purchases of property and equipment in accounts payable		2,645	1,660
Exchange of Senior Secured Term Loan for related party 2L Notes		100,000	0
Debt discount on Senior Secured Term Loan		(1,797)	0
Capital contribution from recognition of delayed draw right asset		690	0
Series A Senior Preferred Stock dividends and redemption value adjustments		80,053	0
Other supplemental disclosures:
Cash paid for interest		52,893	41,617
Cash received from hedging activities		5,380	3,497
Cash (received from) paid for taxes		$ (45)	$ 84

[1] Derivative changes in fair value related to unrealized loss (gain) on cash flow hedges, including the impact of reclassifications.

Overview of the Company

Overview of the Company	12 Months Ended
Overview of the Company	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Overview of the Company	Overview of the Company ATI Physical Therapy, Inc., together with its subsidiaries (herein referred to as “we,” "our," “the Company,” “ATI Physical Therapy” and “ATI”), is a nationally recognized healthcare company, specializing in outpatient rehabilitation and adjacent healthcare services. The Company provides outpatient physical therapy services under the name ATI Physical Therapy and, as of December 31, 2023, had 896 clinics located in 24 states (as well as 18 clinics under management service agreements). ATI Physical Therapy, Inc., a Delaware corporation, was organized in 2020 originally under the name Fortress Value Acquisition Corp. II (herein referred to as "FAII" or "FVAC"). The Company offers a variety of services within its clinics, including physical therapy to treat spine, shoulder, knee and neck injuries or pain; work injury rehabilitation services, including work conditioning and work hardening; hand therapy; and other specialized treatment services. The Company’s direct and indirect wholly-owned subsidiaries include, but are not limited to, Wilco Holdco, Inc., ATI Holdings Acquisition, Inc. and ATI Holdings, LLC. Impact of COVID-19 and CARES Act The coronavirus ("COVID-19") pandemic in the United States resulted in changes to our operating environment. Although the direct impact on our business has decreased since the peak impact in 2020, we continue to closely monitor the remaining impacts from the pandemic including its direct or indirect effects on macroeconomic factors, the labor markets in which we operate, and the physical therapy and broader healthcare landscape. Throughout the duration of the pandemic and declared public health emergency, and continuing hereafter, our priorities have been protecting the health and safety of employees and patients, maximizing the availability of services to satisfy patient needs and improving the operational and financial stability of our business. While we expect the disruption caused by COVID-19 and resulting impacts to diminish over time, we cannot predict the length of such impacts, and if such impacts continue for an extended period, it could have a continued effect on the Company’s results of operations, financial condition and cash flows, which could be material. On March 27, 2020, the Coronavirus Aid, Relief, and Economic Security Act (the “CARES Act”) was signed into law providing reimbursement, grants, waivers and other funds to assist health care providers during the COVID-19 pandemic. The Company realized benefits under the CARES Act including, but not limited to, the receipt of Medicare Accelerated and Advance Payment Program ("MAAPP") funds and deferral of depositing the employer portion of Social Security taxes, interest-free and penalty-free. During the year ended December 31, 2022, the Company applied $12.3 million in MAAPP funds against the outstanding liability at that time. During the year ended December 31, 2022, the remaining obligations related to these benefits were applied and repaid.

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	Basis of Presentation and Summary of Significant Accounting Policies The accompanying consolidated financial statements of the Company were prepared in accordance with U.S. generally accepted accounting principles ("GAAP") and in accordance with the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”). The Company meets the SEC's definition of a "Smaller Reporting Company," and therefore qualifies for the SEC's reduced disclosure requirements for smaller reporting companies. Reverse Stock Split On June 14, 2023, the Company effected a one-for-fifty (1-for-50) reverse stock split of its Class A common stock (the “Reverse Stock Split”). The Reverse Stock Split was approved by the Company’s stockholders at the Company’s 2023 Annual Meeting of Stockholders held on June 13, 2023, and the final reverse split ratio was subsequently approved by the Company’s board of directors on June 14, 2023. The Company's common stock commenced trading on a reverse split-adjusted basis on June 15, 2023. As a result of the Reverse Stock Split, every fifty (50) shares of common stock either issued and outstanding or held as treasury stock were combined into one new share of common stock. Any fractional shares of common stock resulting from the Reverse Stock Split were rounded up to the nearest whole share. All outstanding securities entitling their holders to purchase or acquire shares of common stock, including stock options, warrants, Earnout Shares, Vesting Shares and shares of common stock subject to vesting were adjusted as a result of the Reverse Stock Split, as required by the terms of those securities. The Reverse Stock Split did not change the par value of the common stock or the number of shares authorized for issuance. All information included in these consolidated financial statements and related notes has been adjusted, on a retrospective basis, to reflect the Reverse Stock Split . Liquidity and going concern The accompanying consolidated financial statements have been prepared on a going concern basis, which contemplates the realization of assets and the satisfaction of liabilities in the normal course of business within twelve months after the date that these consolidated financial statements are issued. The Company has negative operating cash flows, operating losses and net losses. For the year ended December 31, 2023, the Company had cash flows used in operating activities of $12.4 million, operating loss of $27.5 million and net loss of $66.1 million. These results are, in part, due to our current capital structure and trends experienced by the Company in recent years including a tight labor market for available physical therapy and other healthcare providers in the workforce, visit volume softness, decreases in rate per visit and increases in interest costs. If results of operations in the coming twelve months do not improve relative to the previous twelve months, the Company is at risk of insufficient funding to meet its obligations as they become due as well as non-compliance with its minimum liquidity financial covenant under its 2022 Credit Agreement. In the Company's consolidated financial statements as of and for the periods ended December 31, 2023 and 2022, these conditions and events continue to raise substantial doubt about the Company's ability to continue as a going concern. In response to these conditions, management plans included refinancing the Company's debt under its 2022 Credit Agreement (as defined in Note 8) and improving operating results and cash flows. On June 15, 2023, the Company completed a debt restructuring transaction under its 2022 Credit Agreement including: (i) a delayed draw new money financing in an aggregate principal amount of $25.0 million, comprised of (A) second lien paid-in-kind ("PIK") convertible notes (the “2L Notes”) and (B) shares of Series B Preferred Stock (as defined in Note 8), which will provide the holder thereof with voting rights such that the holders thereof will have the right to vote on an as-converted basis, (ii) the exchange of $100.0 million of the aggregate principal amount of the term loans under the 2022 Credit Agreement held by certain of the holders of its Series A Senior Preferred Stock (the "Preferred Equityholders") for 2L Notes and Series B Preferred Stock and (iii) certain other changes to the terms of the 2022 Credit Agreement, including modifications of the financial covenants thereunder and relief from the requirements related to the delivery of independent audit reports without a going concern explanatory paragraph. Holders of the 2L Notes will also receive additional 2L Notes upon the in-kind payment of interest on any outstanding 2L Notes. The 2L Notes are convertible into shares of Class A common stock at a fixed conversion price. Additionally, the Company experienced improvements in operations that resulted in reduced levels of operating cash outflows during the year ended December 31, 2023 relative to the same period in the prior year. A continued improvement in business results is necessary as there remains a risk that the Company may fail to meet its minimum liquidity covenant or be unable to fund anticipated cash requirements and obligations as they become due in the future. The Company's plan is to continue its efforts to improve its operating results and cash flow through increases to clinical staffing levels, improvements in clinician productivity, controlling costs and capital expenditures and increases in patient visit volumes, referrals and rate per visit. There can be no assurance that the Company's plan will be successful in any of these respects. If the Company's plan does not result in improvement in these aspects in future periods that results in sufficient cash flow from operations, the Company will need to consider other alternatives, such as raising additional financing, obtaining funds from other sources, disposal of assets, or pursuing other strategic alternatives to improve its business, results of operations and financial condition. There can be no assurance that the Company will be successful in accessing such alternative options or financing if or when needed. Failure to do so could have a material adverse impact on our business, financial condition, results of operations and cash flows, and may lead to events including bankruptcy, reorganization or insolvency. Management plans have not been fully implemented and, as a result, the Company has concluded that management's plans do not alleviate substantial doubt about the Company's ability to continue as a going concern. The consolidated financial statements do not include any adjustments relating to the recoverability and classification of recorded asset amounts or the amounts and classification of liabilities that might result from the outcome of this uncertainty. Principles of consolidation The consolidated financial statements include the financial statements of the Company, its subsidiaries, and entities for which the Company has a controlling financial interest, including variable interest entities ("VIEs") for which the Company is the primary beneficiary. All intercompany balances and transactions have been eliminated in consolidation, and net earnings are reduced by the portion of net earnings attributable to non-controlling interests. Variable interest entities The Company consolidates all variable interest entities where the Company is the primary beneficiary. The Company identifies the primary beneficiary of a VIE as the enterprise that has both: (i) the power to direct the activities of the VIE that most significantly impact the entity's economic performance; and (ii) the obligation to absorb losses or the right to receive benefits of the VIE that could be significant to the entity. The Company may change the original assessment of a VIE upon subsequent events such as the modification of contractual agreements. The Company has an investment in RSFH-ATI Physical Therapy, LLC ("RSFH") that qualifies as a VIE. Based on the provisions of the RSFH agreement, the Company manages the entity and handles all day-to-day operating decisions in exchange for management fees and may receive distributions proportionate with its level of ownership. Accordingly, the Company has the decision-making power over the activities that most significantly impact the entity's economic performance and the obligation to absorb losses or the right to receive benefits that could be significant to the entity. As of December 31, 2023 and 2022, total assets of RSFH were $10.6 million and $10.1 million, respectively, and total liabilities were $4.9 million and $5.0 million, respectively. In general, the assets are available primarily for the settlement of obligations of RSFH. Use of estimates The preparation of the consolidated financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting period. Actual results could differ from those estimates. The effect of any change in estimates will be recognized in the current period of the change. Segment reporting The Company reports segment information based on the management approach. The management approach designates the internal reporting used by management for making decisions and assessing performance as the source of the Company’s reportable segments. All of the Company’s operations are conducted within the United States. Our chief operating decision maker is our Chief Executive Officer, who reviews financial information presented on a consolidated basis for purposes of making decisions, assessing financial performance and allocating resources. We operate our business as one operating segment and therefore we have one reportable segment. Cash, cash equivalents and restricted cash Cash and cash equivalents include all cash balances and highly liquid investments with original maturities of three months or less when issued. Restricted cash consists of cash held as collateral in relation to the Company's corporate card agreement. Restricted cash included within cash and cash equivalents as presented within our consolidated balance sheets as of December 31, 2023 and December 31, 2022, and our consolidated statements of cash flows for the year ended December 31, 2023 and December 31, 2022 was $0.8 million. Accounts receivable The Company's accounts receivable are reported net of contractual allowances, denials allowances and allowances for doubtful accounts. The majority of accounts receivable are due from commercial insurance companies, workers' compensation plans, auto personal injury claims and government health programs, such as Medicare or Medicaid. The Company reports accounts receivable at an amount equal to the consideration the Company expects to receive in exchange for providing healthcare services to its patients. Allowance for doubtful accounts The Company estimates the allowance for doubtful accounts based upon several factors, including the age of the outstanding receivables, the historical experience of collections, the impact of economic conditions and, in some cases, evaluating specific customer accounts for the ability to pay. Management judgment is used to assess the collectability of accounts and the ability of the Company’s customers to pay. Concentrations of business risk The Company provides physical therapy services to a large number of patients who participate in government healthcare programs, resulting in a customer concentration relating to Medicare and Medicaid’s service reimbursement programs. The Company believes that the concentration of credit risk with respect to other patient accounts receivable is limited due to the large number of patients that make up the Company’s patient base and the dispersion across many different insurance companies, preferred provider organizations and individuals. Net patient revenue Net patient revenue consists of revenue for physical therapy services. Net patient revenue is recognized at an amount equal to the consideration the Company expects to receive from third-party payors, patients and others for services rendered when the performance obligations under the terms of the contract are satisfied. There is a contract between the Company and the patient upon each visit resulting in the Company’s performance obligation to provide services to the patient. Generally, the performance obligation is satisfied as the patient receives physical therapy services provided by the Company, as each service provided is distinct and future services rendered are not dependent on previously rendered services. The Company has separate contractual agreements (written or implied) with third-party payors (e.g., insurers, managed care programs, government programs, workers' compensation) that provide for payments to the Company at amounts different from its established rates; the difference between the Company's established rates and amounts paid by third-party payors represent explicit price concessions in the form of contractual allowances or denials allowances. While these agreements are not considered contracts with the customer, they are used for determining the transaction price for services provided to the patients covered by the third-party payors. The payor contracts do not indicate performance obligations of the Company but indicate reimbursement rates for patients who are covered by those payors when the services are provided. To determine the transaction price associated with the contract, the Company includes the estimated effects of any variable consideration, such as contractual allowances, denials allowances and implicit price concessions. When the Company has written payment contracts with third-party payors with negotiated prices for services (contracted payors), the Company determines the transaction price using the negotiated contractual rates to estimate contractual allowances as compared to established rates; additional variable consideration for denial allowances is estimated using a portfolio approach that incorporates whether or not the Company has historical differences from negotiated contractual rates due to non-compliance with contract provisions. When the Company does not have written payment contracts with third-party payors (non-contracted payors), the Company determines the transaction price using a portfolio approach to estimate variable consideration for contractual allowances, denial allowances and implicit price concessions based on historical collections experience for claims with similar characteristics, such as location of service and type of third-party payor, in relation to its established rates. For both contracted and non-contracted payors, any subsequent changes in estimate of the transaction price is recorded as a revenue adjustment. Management believes that calculating at the portfolio level would not differ materially from considering each patient account separately. The Company continually reviews the transaction price estimation process to consider updates to laws and regulations and changes in third-party payor contractual terms that result from contract renegotiations and renewals. Due to complexities involved in determining amounts ultimately due under reimbursement arrangements with third-party payors and government entities, which are often subject to interpretation, the Company may receive reimbursement for healthcare services that is different from the estimates, and such differences could be material. Provision for doubtful accounts For contracted payors, the Company records an estimated provision for doubtful accounts using a portfolio approach based on historical collections experience for claims with similar characteristics, such as location of service and type of third-party payor, at the time net patient revenue is recognized. Any subsequent impairment of the related receivable is recorded as provision for doubtful accounts. The provision for doubtful accounts is included in cost of services in the consolidated statements of operations. When it is determined that a customer account is uncollectible, that balance is written off against the existing allowance. Other revenue Revenue from the ATI Worksite Solutions ("AWS") business is derived from on-site services provided to clients’ employees including injury prevention, rehabilitation, ergonomic assessments and performance optimization. The Company accounts for AWS services as single performance obligations satisfied over time. Revenue from AWS is recognized when obligations under the terms of the contract are satisfied. Revenues are recognized at an amount equal to the consideration the Company expects to receive in exchange for providing services to its clients, which is variable in nature and determined based on the number of hours and respective rate for services provided in a given period. Revenues from Management Service Agreements (“MSA”) are derived from contractual arrangements whereby the Company manages a non-controlled clinic or clinics for third-party owners. The Company does not have any ownership interest in these clinics. The Company accounts for MSA services as single performance obligations satisfied over time. Typically, the transaction price is variable in nature and revenue is determined based on the number of visits conducted at the clinic and recognized when services per the contractual arrangements are performed. Costs, primarily salaries for the Company’s employees, are recorded when incurred. Other revenue includes physical or occupational therapy services and athletic training provided on-site, such as at schools and industrial worksites. Contract terms and rates are agreed to in advance between the Company and the third-parties. Services are typically performed over the contract period, and revenue is recorded as services are performed. If the services are paid in advance, revenue is deferred and recognized as the services are performed. Property and equipment Property and equipment acquired is recorded at cost less accumulated depreciation, except during an acquisition of a business, in which case the assets are initially recorded at fair value. Depreciation is calculated using the straight-line method and is provided in amounts sufficient to attribute the cost of depreciable assets to operations over the estimated useful lives. The approximate useful life of each class of property and equipment is as follows: Equipment 3 - 5 years Furniture & fixtures 5 - 7 years Automobiles 3 - 5 years Software 3 - 5 years Buildings 40 years Leasehold improvements Lesser of lease term or estimated useful lives of the assets (generally 5 - 15 years) Major repairs that extend the useful life of an asset are capitalized to the property and equipment account. Routine maintenance and repairs are charged to rent, clinic supplies, contract labor and other expenses and selling, general and administrative expenses. Gains or losses associated with property and equipment retired or sold are included in earnings. Computer software is included in property and equipment and consists of purchased software and internally developed software. The Company capitalizes application-stage development costs for significant internally developed software projects. Once the software is ready for its intended use, these costs are amortized on a straight-line basis over the software’s estimated useful life. Costs recognized in the preliminary project phase and the post-implementation phase, as well as maintenance and training costs, are expensed as incurred. Impairment of long-lived assets The Company reviews the recoverability of long-lived assets, including operating lease right-of-use assets, whenever events or circumstances occur indicating that the carrying value of the asset may not be recoverable. If the undiscounted cash flows related to the long-lived asset or asset group are not sufficient to recover the remaining carrying value of such asset or asset group, an impairment charge is recognized for the excess carrying amount over the fair value of the asset or asset group. The Company noted triggering events during 2023 and 2022 which resulted in the recording of impairment losses of $5.6 million and $3.0 million for the years ended December 31, 2023 and 2022, respectively. Goodwill and intangible assets Goodwill represents the excess of the purchase price over the fair value of assets acquired and liabilities assumed. The Company accounts for goodwill and indefinite-lived intangible assets under Accounting Standards Codification (“ASC”) Topic 350, Intangibles – Goodwill and Other , which requires the Company to test goodwill and other indefinite-lived assets for impairment annually or whenever events or circumstances indicate that impairment may exist. The Company did not note any triggering events during 2023 that resulted in the recording of an impairment loss, but noted triggering events during 2022 which resulted in the recording of impairment losses. Refer to Note 5 - Goodwill, Trade Name and Other Intangible Assets for further details. Due to the current economic uncertainty resulting from rising interest rates, inflation and other macroeconomic factors, the Company will continue to review the carrying amounts of goodwill and indefinite-lived assets for potential triggering events. The cost of acquired businesses is allocated first to its identifiable assets, both tangible and intangible, based on estimated fair values. Costs allocated to finite-lived identifiable intangible assets are generally amortized on a straight-line basis over the remaining estimated useful lives of the assets. The excess of the purchase price over the fair value of identifiable assets acquired, net of liabilities assumed, is recorded as goodwill. The approximate useful life of each class of intangible asset is as follows: ATI Physical Therapy trade name/trademark Indefinite Non-compete agreements 2 - 5 years Other intangible assets 15 years Goodwill and intangible assets with indefinite lives are not amortized but must be reviewed at least annually for impairment. If the impairment test indicates that the carrying value of the reporting unit exceeds its fair value, then a goodwill impairment loss should be recognized in the consolidated statements of operations in an amount equal to the excess carrying value over fair value. If the impairment test indicates that the carrying value of an intangible asset exceeds its fair value, then an impairment loss should be recognized in the consolidated statements of operations in an amount equal to the excess carrying value over fair value. Fair value is determined using valuation techniques based on estimates, judgments and assumptions the Company believes are appropriate in the circumstances. The Company completed the annual impairment analysis of goodwill as of October 1, 2023 by estimating its fair value using an average of a discounted cash flow analysis and comparable public company analysis. The key assumptions associated with determining the estimated fair value include projected revenue growth rates and EBITDA margins, the terminal growth rate, the discount rate and relevant market multiples. The Company completed the annual impairment analysis of the indefinite-lived intangible asset as of October 1, 2023 by estimating its fair value using the relief from royalty method. The key assumptions associated with determining the estimated fair value include projected revenue growth rates, the royalty rate, the discount rate and the terminal growth rate. Deferred financing costs Original debt issuance discounts and costs incurred related to debt financing are recorded as a reduction to debt and amortized ratably over the term of the related debt agreement, using the effective interest method. Deferred financing costs related to revolving credit facilities are recognized as assets and amortized ratably over the term of the related agreement using the effective interest method. Deferred financing costs are amortized to interest expense, net in the Company’s consolidated statements of operations. The Company recognized amortization of deferred debt issuance costs of $1.6 million and $1.7 million for the years ended December 31, 2023 and 2022, respectively. The Company recognized amortization of original debt issuance discounts of $1.3 million and $1.2 million for the years ended December 31, 2023 and 2022, respectively. Preferred stock Preferred stock is classified as debt, equity or mezzanine equity based on its redemption features. Preferred stock with redemption features outside of the control of the issuer, such as contingent redemption features, is classified as mezzanine equity. Preferred stock with mandatory redemption features is classified as debt. Preferred stock with no redemption features, or redemption features over which the issuer has control, is classified as equity. The Company has preferred stock that is classified as mezzanine equity in the Company's consolidated balance sheets. Refer to Note 11 - Mezzanine and Stockholders' Equity for more information about the Company’s outstanding Series A Senior Preferred Stock. Treasury stock Treasury stock amounts are accounted for under the cost method whereby the entire cost of the acquired stock is recorded as treasury stock. Gains and losses on the subsequent reissuance of shares are credited or charged to paid-in capital in excess of par value using the average-cost method. Warrant liability The Company accounts for its outstanding IPO Warrants in accordance with the guidance contained in ASC Topic 815-40, Derivatives and Hedging - Contracts on an Entity’s Own Equity , and determined that the IPO Warrants do not meet the criteria for equity treatment thereunder. As such, each IPO Warrant must be recorded as a liability and is subject to re-measurement at each balance sheet date. Changes in fair value are recognized in change in fair value of warrant liability in the Company’s consolidated statements of operations. Contingent common shares liability The Company accounts for its potential Earnout Shares and Vesting Shares as a liability in accordance with the guidance in ASC Topic 480, Distinguishing Liabilities from Equity , and ASC Topic 815, Derivatives and Hedging, and is subject to re-measurement at each balance sheet date. Changes in fair value are recognized in change in fair value of contingent common shares liability in the Company’s consolidated statements of operations. Non-controlling interests in consolidated affiliates The consolidated financial statements include all assets, liabilities, revenues and expenses of less-than-100%-owned affiliates where the Company has a controlling financial investment. The Company has separately reflected net income (loss) attributable to the non-controlling interests in net income (loss) in the consolidated statements of operations. Fair value of financial instruments The Company determines fair value measurements used in its consolidated financial statements based upon the exit price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants exclusive of any transaction costs, as determined by either the principal market or the most advantageous market at the measurement date. Inputs used in the valuation techniques to derive fair values are classified based on a three-level hierarchy. The basis for fair value measurements for each level within the hierarchy is described below with Level 1 having the highest priority and Level 3 having the lowest. • Level 1: Observable inputs, which include unadjusted quoted prices in active markets for identical instruments. • Level 2: Observable inputs other than Level 1 inputs, such as quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the instruments. • Level 3: Unobservable inputs in which little or no market data exists, therefore requiring an entity to develop its own assumptions, such as valuations derived from valuation techniques in which one or more significant inputs or significant value drivers are unobservable. Refer to Note 14 - Fair Value Measurements for valuation techniques and inputs related to the Company's financial instruments and share-based liabilities. 2L Notes The guidance in ASC Topic 825, Financial Instruments , provides a fair value option that allows companies to make an irrevocable election of fair value as the initial and subsequent measurement attribute for certain eligible financial assets and liabilities. Unrealized gains and losses on items for which the fair value option has been elected are reported in earnings. The decision to elect the fair value option is determined on an instrument-by-instrument basis, must be applied to an entire instrument and is irrevocable once elected. Assets and liabilities measured at fair value pursuant to this guidance are required to be reported separately in the Company's consolidated balance sheets from those instruments using another accounting method. The 2L Notes are accounted for as a liability in the Company's consolidated balance sheets. The Company has made an irrevocable election to account for the 2L Notes under the fair value option in accordance with ASC Topic 825, Financial Instruments , in lieu of bifurcating certain features in the Second Lien Note Purchase Agreement. As such, the 2L Notes are initially recorded as a liability at estimated fair value and are subject to re-measurement at each balance sheet date with changes in fair value recognized in change in fair value of 2L Notes in the Company’s consolidated statements of operations. Any changes in fair value related to changes in the Company's credit risk is recognized as a component of accumulated other comprehensive income (loss). Income taxes The Company accounts for income taxes in accordance with ASC Topic 740 (“ASC 740”), Income Taxes . Under ASC 740, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and the respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rate is recognized in operations in the period that includes the enactment date. ASC 740 provides guidance on how uncertain tax positions should be recognized, measured, presented and disclosed in the financial statements. ASC 740 requires the evaluation of tax positions taken or expected to be taken in the course of preparing the Company’s tax returns to determine whether the tax positions are more likely than not of being sustained by the applicable tax authority. A tax position that meets the more-likely-than-not recognition threshold is measured and recognized in the consolidated financial statements at the largest amount of benefit that is greater than 50% likely of being realized upon settlement. Tax positions deemed to not meet a more-likely-than-not threshold may not be recognized in the financial statements. The Company reviews these tax uncertainties in light of changing facts and circumstances, such as the progress of tax audits, and if any tax uncertainties were identified, the Company would recognize them accordingly. The liability relating to uncertain tax positions is classified as current in the consolidated balance sheets to the extent the Company anticipates making a payment within one year. The Company did not identify any uncertain tax positions in the years ended December 31, 2023 and December 31, 2022. Interest and penalties associated with income taxes are classified in the income tax expense (benefit) line in the consolidated statements of operations. Cost of services Cost of services consist of salaries specific to the Company’s clinic operations along with rent, clinic supplies expense, depreciation and advertising costs. In addition, cost of services includes the provision for doubtful accounts. Selling, general and administrative expenses Selling, general and administrative expenses consist primarily of wages and benefits for corporate personnel, corporate outside services, marketing costs

Divestitures

Divestitures	12 Months Ended
Divestitures	Dec. 31, 2023
Discontinued Operations and Disposal Groups [Abstract]
Divestitures	Divestitures Clinics held for sale During the fourth quarter of 2023, the Company classified the assets and liabilities of certain clinics as held for sale as a result of the Company's decision to sell the clinics. The divestiture transactions are anticipated to be completed within twelve months. The clinics did not meet the criteria to be classified as discontinued operations. During the fourth quarter of 2022, the Company classified the assets and liabilities of certain clinics as held for sale as a result of the Company's decision to sell the clinics. The divestiture transactions were anticipated to be completed within twelve months. The clinics did not meet the criteria to be classified as discontinued operations. During the first quarter of 2023, the Company completed a portion of its anticipated divestiture transactions, which were immaterial. During the second quarter of 2023, the Company concluded the remaining anticipated divestiture transactions were no longer probable due to the Company's decision to retain the clinics. As a result, the remaining assets and liabilities previously classified as held for sale were reclassified as held and used into the respective line items within the consolidated balance sheet. Major classes of assets and liabilities classified as held for sale as of December 31, 2023 and December 31, 2022 were as follows (in thousands): December 31, 2023 December 31, 2022 Accounts receivable, net $ — $ 486 Prepaid expenses — 23 Property and equipment, net 674 1,113 Operating lease right-of-use assets 1,382 1,929 Goodwill, net — 3,192 Other non-current assets — 12 Total assets held for sale $ 2,056 $ 6,755 Accounts payable $ — $ 22 Accrued expenses and other liabilities — 201 Current portion of operating lease liabilities 357 685 Operating lease liabilities 1,421 1,706 Total liabilities held for sale $ 1,778 $ 2,614

Revenue from Contracts with Cus

Revenue from Contracts with Customers	12 Months Ended
Revenue from Contracts with Customers	Dec. 31, 2023
Revenue from Contract with Customer [Abstract]
Revenue from Contracts with Customers	Revenue from Contracts with Customers The following table disaggregates net revenue by major service line for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Net patient revenue $ 636,095 $ 575,940 ATI Worksite Solutions (1) 37,219 35,515 Management Service Agreements (1) 14,831 12,857 Sports Medicine and other revenue (1) 10,871 11,359 $ 699,016 $ 635,671 (1) ATI Worksite Solutions, Management Service Agreements and Sports Medicine and other revenue are included within other revenue on the face of the consolidated statements of operations. The following table disaggregates net patient revenue for each associated payor class as a percentage of total net patient revenue for the periods indicated below: Year Ended December 31, 2023 December 31, 2022 Commercial 58.6 % 57.6 % Government 23.2 % 24.2 % Workers’ compensation 11.7 % 12.4 % Other (1) 6.5 % 5.8 % 100.0 % 100.0 % (1) Other is primarily comprised of net patient revenue related to auto personal injury reimbursement.

Goodwill, Trade Name and Other

Goodwill, Trade Name and Other Intangible Assets	12 Months Ended
Goodwill, Trade Name and Other Intangible Assets	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill, Trade Name and Other Intangible Assets	Goodwill, Trade Name and Other Intangible Assets Our indefinite-lived intangible asset consists of the ATI trade name. We test the indefinite-lived intangible asset for impairment on an annual basis as of October 1. While the Company concluded that no indefinite-lived intangible asset impairment existed at the time of annual impairment tests performed for the years ended December 31, 2023 and 2022, the Company noted separate interim triggering events during 2022 which resulted in the recording of impairment losses. For the annual impairment test performed in 2022, no impairment existed as of October 1 since impairment was recorded on September 30. The Company has one reporting unit for purposes of the Company’s goodwill impairment test, which is completed as of October 1. While the Company concluded that no goodwill impairment existed at the time of the annual impairment test performed for the years ended December 31, 2023 and 2022, the Company noted separate interim triggering events during 2022 which resulted in the recording of impairment losses. For the annual impairment test performed in 2022, no impairment existed as of October 1 since impairment was recorded on September 30. Changes in the carrying amount of goodwill during the current year consisted of the following (in thousands): Goodwill at December 31, 2022 (1) $ 286,458 Impairment charges (2) — Reclassifications to held and used 3,192 Goodwill at December 31, 2023 (1) $ 289,650 (1) Net of accumulated impairment losses of $1,045.7 million. (2) The Company did not note any triggering events during the year ended December 31, 2023 that resulted in the recording of an impairment loss. The table below summarizes the Company’s carrying amount of trade name and other intangible assets at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Gross intangible assets: ATI trade name (1) $ 245,000 $ 245,000 Non-compete agreements 2,395 2,395 Other intangible assets 640 640 Accumulated amortization: Accumulated amortization – non-compete agreements (1,807) (1,126) Accumulated amortization – other intangible assets (370) (327) Total trade name and other intangible assets, net $ 245,858 $ 246,582 (1) Not subject to amortization. Amortization expense for the years ended December 31, 2023 and 2022 was immaterial. The Company estimates that amortization expense related to intangible assets will be immaterial over the next five fiscal years and thereafter. Interim impairment testing during 2022 During the quarters ended March 31, 2022, June 30, 2022, September 30, 2022 and December 31, 2022, the Company identified interim triggering events as a result of factors including potential changes in discount rates and decreases in share price. The Company determined that the combination of these factors constituted interim triggering events that required further analysis with respect to potential impairment to goodwill, trade name indefinite-lived intangible and other assets. The Company assessed its long-lived asset groups, including operating lease right-of-use assets that were evaluated based on clinic-specific cash flows and clinic-specific market factors, noting no material impairment. As it was determined that it was more likely than not that the fair value of our trade name indefinite-lived intangible asset was below its carrying value, the Company performed an interim quantitative impairment test as of the March 31, 2022, June 30, 2022, September 30, 2022 and December 31, 2022 balance sheet dates. The Company utilized the relief from royalty method to estimate the fair value of the trade name indefinite-lived intangible asset. The key assumptions associated with determining the estimated fair value included projected revenue growth rates, the royalty rate, the discount rate and the terminal growth rate. As a result of the analyses, during the year ended December 31, 2022, the Company recognized $164.4 million in non-cash interim impairments in goodwill, intangible and other asset impairment charges As it was determined that it was more likely than not that the fair value of our single reporting unit was below its carrying value, the Company performed an interim quantitative impairment test with respect to goodwill. In order to determine the fair value of our single reporting unit, the Company utilized an average of a discounted cash flow analysis and comparable public company analysis. The key assumptions associated with determining the estimated fair value included projected revenue growth rates, earnings before interest, taxes, depreciation and amortization ("EBITDA") margins, the terminal growth rate, the discount rate and relevant market multiples. As a result of the analyses, during the year ended December 31, 2022, the Company recognized $318.9 million in non-cash interim impairments in goodwill, intangible and other asset impairment charges in its consolidated statements of operations, which represented the difference between the estimated fair value of the Company’s single reporting unit and its carrying value. Fair value determinations require considerable judgment and are sensitive to changes in underlying assumptions, estimates and market factors. Estimating the fair value of the Company’s reporting unit and the indefinite-lived intangible asset requires us to make assumptions and estimates regarding our future plans, as well as industry, economic, and regulatory conditions. These assumptions and estimates include projected revenue growth rates and EBITDA margins, terminal growth rates, discount rates, relevant market multiples, royalty rates and other market factors. If current expectations of future growth rates, margins and cash flows are not met, or if market factors outside of our control change significantly, including discount rates, relevant market multiples, company share price and other market factors, then our reporting unit or the indefinite-lived intangible asset might become impaired in the future, negatively impacting our operating results and financial position. Factors that could result in the cash flows being lower than the current estimates include decreased revenue caused by unforeseen changes in the healthcare market or the Company's business, or the inability to achieve the estimated operating margins in the forecasts due to unforeseen factors. Additionally, changes in the broader economic environments could cause changes to the estimated discount rates and comparable company valuation indicators which may impact the estimated fair values. As the carrying amounts of goodwill and the Company’s trade name indefinite-lived intangible asset were impaired as of December 31, 2022 and written down to fair value, those amounts are more susceptible to an impairment risk if there are unfavorable changes in assumptions and estimates.

Property and Equipment

Property and Equipment	12 Months Ended
Property and Equipment	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment consisted of the following at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Equipment $ 37,980 $ 38,102 Furniture and fixtures 14,311 17,215 Leasehold improvements 178,888 191,182 Automobiles 4 19 Computer equipment and software 108,749 102,651 Construction-in-progress 2,134 3,727 342,066 352,896 Accumulated depreciation and amortization (241,644) (229,206) Property and equipment, net (1) $ 100,422 $ 123,690 (1) Excludes $0.7 million and $1.1 million reclassified as held for sale as of December 31, 2023 and December 31, 2022, respectively. Refer to Note 3 - Divestitures for additional information. Property and equipment includes internally developed computer software costs in the amount of $66.1 million and $64.3 million as of December 31, 2023 and 2022, respectively. The related amortization expense was $7.7 million and $8.2 million for the years ended December 31, 2023 and 2022, respectively. Depreciation and amortization expense is recorded within rent, clinic supplies, contract labor and other and selling, general and administrative expenses within the consolidated statements of operations, depending on the use of the underlying fixed assets. The depreciation and amortization expense recorded in rent, clinic supplies, contract labor and other relates to revenue-generating assets, which primarily includes clinic leasehold improvements and therapy equipment. The depreciation and amortization expense included in selling, general and administrative expenses is related to infrastructure items, such as corporate leasehold improvements, computer equipment and software. The following table presents the amount of depreciation and amortization expense related to property and equipment recorded in rent, clinic supplies, contract labor and other and selling, general and administrative expenses in the Company’s consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Rent, clinic supplies, contract labor and other $ 25,179 $ 27,429 Selling, general and administrative expenses 11,509 12,417 Total depreciation expense $ 36,688 $ 39,846

Accrued Expenses and Other Liab

Accrued Expenses and Other Liabilities	12 Months Ended
Accrued Expenses and Other Liabilities	Dec. 31, 2023
Payables and Accruals [Abstract]
Accrued Expenses and Other Liabilities	Accrued Expenses and Other Liabilities Accrued expenses and other liabilities consisted of the following at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Salaries and related costs $ 37,630 $ 28,949 Accrued legal settlement (1) 21,324 1,015 Credit balances due to patients and payors 7,712 6,117 Accrued professional fees 4,146 4,536 Accrued interest 4,913 762 Accrued occupancy costs 2,593 2,410 Accrued contract labor 2,255 4,483 Other payables and accrued expenses 7,862 5,400 Total $ 88,435 $ 53,672 (1) Includes estimated liability of $20.0 million related to settlement agreement in principle as of December 31, 2023. Refer to Note 17 - Commitments and Contingencies for additional information.

Borrowings

Borrowings	12 Months Ended
Borrowings	Dec. 31, 2023
Debt Disclosure [Abstract]
Borrowings	Borrowings Long-term debt, net consisted of the following at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Senior Secured Term Loan (1, 2) (due February 24, 2028) $ 410,048 $ 503,481 Revolving Loans (3) (due February 24, 2027) 38,450 48,200 Less: unamortized debt issuance costs (7,395) (11,137) Less: unamortized original issue discount (7,525) (8,944) Total debt, net 433,578 531,600 Less: current portion of long-term debt — — Long-term debt, net $ 433,578 $ 531,600 (1) Interest rate of 12.7% and 12.1% at December 31, 2023 and December 31, 2022, respectively, with interest payable in designated installments at a variable interest rate. The effective interest rate for the Senior Secured Term Loan was 13.9% and 13.1% at December 31, 2023 and December 31, 2022, respectively. (2) The Company has paid a portion of its interest in-kind on its Senior Secured Term Loan by capitalizing and adding such interest to the principal amount of the debt. As of December 31, 2023 and December 31, 2022, the Company has recognized total paid-in-kind interest in the amount of $10.0 million and $3.5 million, respectively. (3) Weighted average interest rate of 9.5% and 8.3% at December 31, 2023 and December 31, 2022, respectively, with interest payable in designated installments at a variable interest rate. 2L Notes due to related parties, at fair value consisted of the following at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 2L Notes due to related parties, at fair value $ 79,472 $ — 2023 Debt Restructuring Transaction On June 15, 2023 (the "Closing Date"), the Company completed a debt restructuring transaction to improve the Company's liquidity (the "2023 Debt Restructuring"). On the Closing Date, certain previously executed agreements became effective, including (i) Amendment No. 2 to the Credit Agreement, (ii) a Second Lien Note Purchase Agreement and (iii) certain other definitive agreements relating to the 2023 Debt Restructuring (such documents referred to collectively as the "Signing Date Definitive Documents"). As part of the 2023 Debt Restructuring, the Company exchanged a principal amount of $100.0 million of the $507.8 million then outstanding Senior Secured Term Loan for an equal amount of 2L Notes, which are convertible into shares of the Company's common stock, stapled with a number of shares of Series B Preferred Stock (the "Series B Preferred Stock"), which represent voting interests only. The exchange was consummated through the Intercreditor and Subordination Agreement and Second Lien Note Purchase Agreement dated April 17, 2023 (the "Signing Date"). The Company accounted for the exchange as a debt extinguishment and recognized $0.4 million in loss on debt extinguishment during the year ended December 31, 2023. The loss on debt extinguishment consisted of various offsetting components, including the derecognition of $4.3 million of unamortized deferred financing costs and original issue discount on the Senior Secured Term Loan and the recognition of $0.7 million of fair value premium at issuance on the 2L Notes, offset by the recognition of $2.8 million in delayed draw right assets related to the commitment provided by certain lenders and the recognition of $1.8 million of incremental original issue discount on the Senior Secured Term Loan. The loss on debt extinguishment associated with the 2023 Debt Restructuring has been reflected in other expense, net in the consolidated statements of operations. Amendment No. 2 to the Credit Agreement Pursuant to Amendment No. 2 to the Credit Agreement, the terms of the remaining unexchanged $407.8 million principal amount of the Senior Secured Term Loan as of the Signing Date were revised to: (i) increase the interest rate in the form of paid-in-kind interest by 1.0% per annum until the achievement of certain financial metrics, (ii) reset the prepayment premiums with respect to any repayment of the Senior Secured Term Loan, and (iii) amend certain covenants. At the completion of the 2023 Debt Restructuring, $391.0 million principal of amended Senior Secured Term Loan was outstanding with HPS Investment Partners, LLC (“HPS”), $16.3 million principal was outstanding with Onex Credit Partners, LLC (“Onex”), $0.3 million principal was outstanding with Knighthead Capital Management, LLC (“Knighthead”), and the remaining $0.2 million principal was outstanding with Marathon Asset Management LP (“Marathon”). Additionally, the terms of the Company's Revolving Loans were revised to increase the cash interest rate by 1.0% until the achievement of certain financial metrics. Amendment No. 2 to the Credit Agreement also provides, among other terms, (i) a reduction of the thresholds applicable to the minimum liquidity financial covenant under the 2022 Credit Agreement for certain periods, (ii) a waiver of the requirement to comply with the Secured Net Leverage Ratio financial covenant under the 2022 Credit Agreement for the fiscal quarters ending June 30, 2024, September 30, 2024 and December 31, 2024 and a modification of the levels and certain component definitions applicable thereto in the fiscal quarters ending after December 31, 2024, (iii) an extension of the minimum liquidity financial covenant for the fiscal quarters in which the Secured Net Leverage Ratio financial covenant was waived, (iv) a waiver of the requirement for the Company to deliver audited financial statements without a going concern explanatory paragraph for the years ended December 31, 2022, December 31, 2023, and December 31, 2024, and (v) board representation and observer rights and other changes to the governance of the Company. Based on the results of the cash flow tests and requirements pursuant to ASC Topic 470, Debt , the Company accounted for the impacts of Amendment No. 2 to the Credit Agreement related to the amount held by HPS as a modification, and the impacts related to the amounts held by Onex, Knighthead, and Marathon as an extinguishment. As part of the 2023 Debt Restructuring, the Company recognized $1.8 million of incremental original issue discount on the Senior Secured Term Loan related to lenders treated under extinguishment accounting. Second Lien Note Purchase Agreement and Designation of Series B Preferred Stock Knighthead, Marathon, and Onex collectively exchanged a principal amount of $100.0 million of Senior Secured Term Loan for $100.0 million of 2L Notes stapled with a number of shares of Series B Preferred Stock. Of the $100.0 million of 2L Notes issued, approximately $50.8 million were issued to Knighthead, $40.4 million were issued to Marathon, and $8.8 million were issued to Onex. The 2L Notes are subordinated in right of payment and lien priority to the 2022 Credit Facility and mature on August 24, 2028, unless earlier converted, accrue interest at an annual rate of 8.0% payable in-kind on a quarterly basis in the form of additional 2L Notes, and are convertible into shares of common stock, at the holder’s option, at a fixed conversion price of $12.50, subject to certain adjustments in the agreement (the "Conversion Price"). Upon conversion of the 2L Notes, the Company shall deliver to the holder a number of shares of common stock equal to (i) the principal amount of such 2L Notes plus any accrued and unpaid interest divided by (ii) the Conversion Price. The 2L Notes are effectively stapled with one share of the Company’s Series B Preferred Stock for every $1,000 principal amount of the 2L Notes. The Series B Preferred Stock represents voting rights only, with the number of votes being equal to the number of shares of common stock that each share of Series B Preferred Stock would convert into at a conversion price of $12.87 per share (the "Voting Rights Conversion Price"). Additional voting rights accrue to the lenders through the deemed issuance of the annual 8.0% paid-in-kind 2L Notes with stapled shares of Series B Preferred Stock. The Series B Preferred Stock does not have any dividend or redemption rights. Upon conversion of 2L Notes to common stock, the stapled shares of Series B Preferred Stock would be canceled in an amount commensurate with the portion of 2L Notes converted. Based on the voting rights associated with the Series B Preferred Stock attached to the 2L Notes as well as other terms to the 2023 Debt Restructuring, the Company determined that Knighthead, Marathon, and Onex became related parties on the Closing Date. On the Closing Date, an additional $3.2 million of 2L Notes with stapled Series B Preferred Stock were issued as part of the First Amendment to the Second Lien Note Purchase Agreement. The terms of the issued 2L Notes and Series B Preferred Stock are the same as those that were subject to the exchange. The following table presents approximate changes in outstanding shares of Series B Preferred Stock since the Closing Date and associated equivalent common stock voting rights at the end of the period (in thousands): December 31, 2023 Series B Preferred Stock, shares at Closing Date 103 Increase (decrease) in shares during period 5 Series B Preferred Stock, shares at end of period 108 Common stock voting rights, as converted basis (1) 8,377 (1) Represents approximate shares of Series B Preferred Stock outstanding at end of period, times $1,000, divided by the contractual Voting Rights Conversion Price of $12.87 per share. On or after the second anniversary of the Closing Date and subject to certain conditions, the Company may, at its option, elect to convert (a “Forced Conversion”) a portion of the outstanding 2L Notes into the number of shares of common stock based on the Conversion Price then in effect. The 2L Notes are accounted for as a liability in the Company's consolidated balance sheets. The Company has made an irrevocable election to account for the 2L Notes under the fair value option in accordance with ASC Topic 825, Financial Instruments , in lieu of bifurcating certain features in the Second Lien Note Purchase Agreement. As such, the 2L Notes are initially recorded as a liability at estimated fair value and are subject to re-measurement at each balance sheet date with changes in fair value recognized in the Company's statements of operations. The interest cost associated with the 2L Notes is accounted for as part of the change in fair value of the 2L Notes. As a result of applying the fair value option, direct costs and fees related to the issuance of the 2L Notes were expensed as incurred. As of December 31, 2023, the principal amount and estimated fair value of the 2L Notes were approximately $107.8 million and $79.5 million, respectively. Refer to Note 14 - Fair Value Measurements for further details on the fair value of the 2L Notes. Additionally, as of December 31, 2023, the effective interest rate on the 2L Notes was 8.0%. The following table presents changes in the principal amount of the 2L Notes since the Closing Date (in thousands): December 31, 2023 2L Notes, principal amount at Closing Date $ 103,243 Paid-in-kind interest added during period 4,569 2L Notes, principal amount at end of period $ 107,812 As of December 31, 2023, of the 2L Notes principal outstanding and due to related parties, approximately $54.7 million, $43.6 million and $9.5 million were outstanding with Knighthead, Marathon, and Onex, respectively. Delayed Draw Right The Company also has the right to cause to be issued to Knighthead, Marathon and Caspian Capital L.P. ("Caspian") (collectively the "Delayed Draw Purchasers") an additional $25.0 million of aggregate principal in the form of 2L Notes under its delayed draw right ("Delayed Draw Right”), which is governed by the Second Lien Note Purchase Agreement. If drawn, the notes under the Delayed Draw Right will be subject to the same terms as the convertible 2L Notes with associated shares of Series B Preferred Stock allowing for voting rights on an as-converted basis prior to conversion. The right to draw will terminate approximately 18 months after the Closing Date. The Company may request two draws in an amount of $12.5 million each, separately or together, subject to, for each draw, (a) projected liquidity at any time during the 6-month period following the date of the relevant draw being below certain thresholds, and (b) the consent of the board of directors. Upon issuance, the Company accounted for the Delayed Draw Right as an asset at fair value, which represents the Company's option to draw funds subject to certain conditions. For Knighthead's and Marathon's portion of the Delayed Draw Right, the asset was recognized as part of the calculation of loss on debt extinguishment. For Caspian, the Delayed Draw Right was recognized as a capital contribution as there was no previous lender relationship with the Company with respect to the Senior Secured Term Loan. At the Closing Date, the Company recognized approximately $3.5 million in Delayed Draw Right assets, which is included in other current assets on the Company's consolidated balance sheets. Subsequently, the asset will be monitored for impairment. As of December 31, 2023, no impairment indicators were identified. On January 30, 2024, the Company issued $25.0 million of aggregate principal in the form of 2L Notes under its Delayed Draw Right, which are subject to the same terms as the convertible 2L Notes and associated shares of Series B Preferred Stock allowing for voting rights on an as-converted basis prior to conversion. Approximately $12.0 million, $8.0 million, and $5.0 million of the 2L Notes were issued to Knighthead, Marathon and Caspian, respectively. 2022 Credit Agreement On February 24, 2022 (the "Refinancing Date"), the Company entered into various financing arrangements to refinance its previous long-term debt (the "2022 Debt Refinancing"). As part of the 2022 Debt Refinancing, ATI Holdings Acquisition, Inc. (the "Borrower"), an indirect subsidiary of ATI Physical Therapy, Inc., entered into a credit agreement among the Borrower, Wilco Intermediate Holdings, Inc. ("Holdings"), as loan guarantor, Barclays Bank PLC, as administrative agent and issuing bank, and a syndicate of lenders (the "2022 Credit Agreement"). The 2022 Credit Agreement provides a $550.0 million credit facility (the "2022 Credit Facility") that is comprised of a $500.0 million senior secured term loan (the "Senior Secured Term Loan") which was fully funded at closing and a $50.0 million "super priority" senior secured revolver (the "Revolving Loans") with a $10.0 million letter of credit sublimit. The 2022 Credit Facility refinanced and replaced the Company's prior credit facility for which Barclays Bank PLC served as administrative agent for a syndicate of lenders. The Company paid $555.0 million to settle its previous term loan (the "2016 First Lien Term Loan"). The Company accounted for the transaction as a debt extinguishment and recognized $2.8 million in loss on debt extinguishment during the year ended December 31, 2022 related to the derecognition of the remaining unamortized deferred financing costs and unamortized original issue discount in conjunction with the debt repayment. The loss on debt extinguishment associated with the repayment of the 2016 First Lien Term Loan has been reflected in other expense, net in the consolidated statements of operations. In connection with the 2022 Debt Refinancing, the Company also entered into a preferred stock purchase agreement, consisting of senior preferred stock with detachable warrants to purchase common stock for an aggregate stated value of $165.0 million (collectively, the “Preferred Stock Financing”). See Note 11 - Mezzanine and Stockholders' Equity for further information regarding the Preferred Stock Financing. The Company capitalized debt issuance costs totaling $12.5 million related to the 2022 Credit Facility as well as an original issue discount of $10.0 million, which are amortized over the terms of the respective financing arrangements. Senior Secured Term Loan The Senior Secured Term Loan matures on February 24, 2028 and bears interest, at the Company's election, at a base interest rate of the Alternate Base Rate ("ABR"), as defined in the agreement, plus an applicable credit spread, or the Adjusted Term Secured Overnight Financing Rate ("SOFR"), as defined in the agreement, plus an applicable credit spread. The credit spread is determined based on a pricing grid and the Company's Secured Net Leverage Ratio. The Company was able to elect to pay 2.0% interest in-kind at a 0.5% premium during the first year under the agreement. The Company elected to pay a portion of its interest in-kind beginning in the third quarter of 2022 through the completion of the first year under the agreement. As of December 31, 2023, borrowings on the Senior Secured Term Loan bear interest at 12.7%, consisting of 12-month SOFR, subject to a 1.0% floor, plus a credit spread of 7.25%. As of December 31, 2023, the effective interest rate on the Senior Secured Term Loan was 13.9% and the outstanding principal amount was $410.0 million, of which $17.0 million was due to related parties and is primarily attributable to Onex. Beginning in October 2023, the Company is no longer incurring the incremental 1.0% paid-in-kind interest on its Senior Secured Term Loan based on its achievement of the required financial metrics under the terms of the 2023 Debt Restructuring. Revolving Loans The Revolving Loans are subject to a maximum borrowing capacity of $50.0 million and mature on February 24, 2027. Borrowings on the Revolving Loans bear interest, at the Company's election, at a base interest rate of the ABR, as defined in the agreement, plus an applicable credit spread, or the Adjusted Term SOFR Rate, as defined in the agreement, plus an applicable credit spread. The credit spread is determined based on a pricing grid and the Company's Secured Net Leverage Ratio. In December 2022, the Company drew $48.2 million in Revolving Loans. During 2023, the Company repaid approximately $44.8 million in Revolving Loans and drew an additional $35.0 million in Revolving Loans. As of December 31, 2023, $38.5 million in Revolving Loans were outstanding and bearing interest at a weighted average rate of 9.5%, consisting of 3-month SOFR plus a credit spread of approximately 4.1%. Beginning in October 2023, the Company is no longer incurring the incremental 1.0% interest on its Revolving Loans based on its achievement of the required financial metrics under the terms of the 2023 Debt Restructuring. Commitment fees on the Revolving Loans are payable quarterly at 0.5% per annum on the daily average undrawn portion for the quarter and are expensed as incurred. The balances of unamortized issuance costs related to the Revolving Loans were $0.5 million as of December 31, 2023, and $0.6 million as of December 31, 2022. The 2022 Credit Facility and 2L Notes are guaranteed by certain of the Company’s subsidiaries and are secured by substantially all of the assets of Holdings, the Borrower and the Borrower’s wholly-owned subsidiaries, including a pledge of the stock of the Borrower, in each case, subject to customary exceptions. Pursuant to the terms of the Intercreditor and Subordination Agreement, the 2L Notes (and the guarantees thereof) will rank junior in right of payment to the obligations under the 2022 Credit Agreement, and the liens on the collateral securing the 2L Notes will rank junior to the liens on such collateral securing the obligations under the 2022 Credit Agreement. The 2022 Credit Agreement contains customary covenants and restrictions, including financial and non-financial covenants. In accordance with Amendment No. 2 to the Credit Agreement, the financial covenants require the Company to maintain $30.0 million of minimum liquidity, as defined in the agreement, at each test date through the first quarter of 2023, $25.0 million of minimum liquidity for the second quarter of 2023, $15.0 million of minimum liquidity through the fourth quarter of 2023 and $10.0 million of minimum liquidity through the fourth quarter of 2024. Additionally, beginning in the first quarter of 2025, the Company must maintain a Secured Net Leverage Ratio, as defined in the agreement, not to exceed 11.00:1.00. The net leverage ratio covenant decreases each subsequent quarter through the second quarter of 2026 to 7.00:1.00, which remains applicable through maturity. The financial covenants are tested as of each fiscal quarter end for the respective periods. As of December 31, 2023, the Company is in compliance with its minimum liquidity financial covenant. The 2022 Credit Facility contains customary representations and warranties, events of default, reporting and other affirmative covenants and negative covenants, including requirements related to the delivery of independent audit reports without a going concern explanatory paragraph beginning with the report covering fiscal year 2025, limitations on indebtedness, liens, investments, negative pledges, dividends, junior debt payments, fundamental changes and asset sales and affiliate transactions. The Second Lien Note Purchase Agreement includes affirmative and negative covenants (other than financial covenants) that are substantially consistent with the 2022 Credit Agreement, as well as customary events of default. Failure to comply with the 2022 Credit Facility and Second Lien Note Purchase Agreement covenants and restrictions could result in an event of default under the respective borrowing agreements, subject to customary cure periods. In such an event, all amounts outstanding under the 2022 Credit Facility and Second Lien Note Purchase Agreement, together with any accrued interest, could then be declared immediately due and payable. Under the 2022 Credit Facility, the Company may be required to make certain mandatory prepayments upon the occurrence of certain events, including: an event of default, a prepayment asset sale or receipt of net insurance proceeds in excess of $10.0 million, or excess cash flows exceeding certain thresholds. A prepayment asset sale includes dispositions at fair market value, and net insurance proceeds is generally defined as insurance proceeds received on a covered loss or as a result of assets taken under the power of eminent domain, net of costs related to the matter. The Company had letters of credit totaling $6.5 million and $1.8 million under the letter of credit sub-facility on the Revolving Loans as of December 31, 2023 and December 31, 2022, respectively. The letters of credit auto-renew on an annual basis and are pledged to insurance carriers as collateral. Aggregate maturities of the Company's borrowings at December 31, 2023 are as follows (in thousands): 2024 $ — 2025 — 2026 — 2027 38,450 2028 517,860 Thereafter — Total future maturities (1) 556,310 Unamortized original issue discount and debt issuance costs (14,920) 2L Notes due to related parties, principal amount (1, 2) (107,812) Long-term debt, net (1) $ 433,578 (1) Excludes any contractual paid-in-kind interest that may be accrued and added to the principal amounts between now and the respective maturity dates. (2) The principal amount of the 2L Notes differs from the estimated fair value presented on the consolidated balance sheet. Refer to Note 14 - Fair Value Measurements for further details on the fair value of the 2L Notes.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
Employee Benefit Plans	Dec. 31, 2023
Retirement Benefits [Abstract]
Employee Benefit Plans	Employee Benefit Plans The Company maintains a defined contribution 401(k) retirement plan for its full-time employees. The plan allows all participants to make elective pretax contributions of up to 100% of their compensation, up to a maximum amount as limited by law. The Company makes matching contributions to the plan on behalf of the employee in the amount of 50% of the first 6% of the contributing participant’s elective deferral contribution. Matching contributions to the plan were $5.4 million and $4.9 million for the years ended December 31, 2023 and 2022, respectively. The following table presents the Company’s matching contributions to the plan recorded in salaries and related costs and selling, general and administrative expenses in the consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Salaries and related costs $ 4,803 $ 4,374 Selling, general and administrative expenses 592 559 Total $ 5,395 $ 4,933

Share-Based Compensation

Share-Based Compensation	12 Months Ended
Share-Based Compensation	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Share-Based Compensation	Share-Based Compensation The Company recognizes compensation expense for all share-based compensation awarded to employees, net of forfeitures, using a fair value-based method. The grant-date fair value of each award is amortized to expense on a straight-line basis over the award’s vesting period. Compensation expense associated with share-based awards is included in salaries and related costs and selling, general and administrative expenses in the accompanying consolidated statements of operations, depending on whether the award recipient is a clinic-level or corporate employee, respectively. Share-based compensation expense is adjusted for forfeitures as incurred. Wilco Acquisition, LP 2016 Equity Incentive Plan Prior to the Business Combination transaction on June 16, 2021 between Wilco Holdco, Inc. and FAII (the "Business Combination"), Wilco Acquisition, LP was the parent company of Wilco Holdco, Inc. and its subsidiaries. In 2016, the Company adopted the Wilco Acquisition, LP 2016 Equity Incentive Plan (the “2016 Plan”) under which, prior to the Business Combination, it granted profit interests of Wilco Acquisition, LP in the form of Incentive Common Units ("ICUs"), to members of management, key employees and independent directors of Wilco Acquisition, LP and its subsidiaries. The ICUs granted consisted of awards subject to service-based vesting and performance-based vesting. Following the closing of the Business Combination, holders of ICUs were entitled to a distribution of a number of Class A common shares of ATI Physical Therapy, Inc. based on the distribution priorities under the Wilco Acquisition, LP limited partnership agreement. The shares related to vested service-based ICUs were distributed as unrestricted Class A common shares of ATI. The shares related to unvested service-based ICUs were distributed as restricted Class A common shares of ATI eligible to vest over the shorter of: (a) the existing vesting schedule applicable to the underlying ICUs, or (b) in installments on each quarterly anniversary of the closing over three years post-closing of the Business Combination, subject to the grantee's continued service through each vesting date. The shares related to performance-based ICUs were distributed to holders as restricted Class A common shares of ATI eligible to vest in installments on each quarterly anniversary of the closing over the shorter of: (a) the eight-year period from the original grant date of the underlying ICUs, or (b) three years post-closing of the Business Combination, subject to the grantee’s continued service through each vesting date. Based on the terms of the performance-based ICUs, the performance-based awards follow the treatment of an initial public offering ("IPO") as a result of the Business Combination and, therefore, converted to service-based vesting requirements. Pursuant to the 2016 Plan, total share-based compensation expense recognized in the years ended December 31, 2023 and 2022 was $0.6 million and $1.1 million, respectively. There were no awards granted under the 2016 Plan during the year ended December 31, 2023. For the year ended December 31, 2023, vestings related to shares distributed to holders of ICUs were immaterial, and forfeitures related to shares distributed to holders of ICUs were immaterial. As of December 31, 2023, the remaining unvested restricted shares distributed to holders of ICUs and unrecognized compensation expense were immaterial. ATI 2021 Equity Incentive Plan The Company adopted the ATI Physical Therapy 2021 Equity Incentive Plan (the "2021 Plan") under which it may grant equity interests of ATI Physical Therapy, Inc., in the form of stock options, stock appreciation rights, restricted stock awards and restricted stock units, to members of management, key employees and independent directors of the Company and its subsidiaries. The Compensation Committee is authorized to make grants and to make various other decisions under the 2021 Plan. The maximum number of shares reserved for issuance under the 2021 Plan is approximately 1.2 million. As of December 31, 2023, approximately 0.2 million shares were available for future grant. Stock options The Company grants stock options to members of management, key employees and independent directors. Stock options typically vest in equal annual installments over a service period ranging from three Pursuant to the 2021 Plan, total share-based compensation expense related to stock options recognized in the years ended December 31, 2023 and 2022 was approximately $1.5 million and $1.3 million, respectively. The following table summarizes the activity of stock options for the year ended December 31, 2023 (aggregate intrinsic value in thousands): Number of Options Weighted-Average Exercise Price Weighted-Average Contractual Term (in years) Aggregate Intrinsic Value Outstanding, January 1, 2023 106,495 $ 92.14 9.1 $ — Granted — — N/A N/A Adjustments 2,676 87.00 N/A N/A Exercised — — N/A — Forfeited/Cancelled (9,886) 96.46 N/A N/A Outstanding, December 31, 2023 99,285 $ 91.73 8.2 $ — Exercisable, December 31, 2023 32,181 $ 99.93 8.1 $ — Unvested, December 31, 2023 67,104 $ 87.80 8.2 $ — The fair values of each stock option granted was determined using the Black-Scholes option-pricing model. As the Company does not have sufficient historical share option exercise experience for such "plain-vanilla" awards, the expected option term was determined using the simplified method, which is the average of the option's vesting and contractual term. Volatility is measured using the historical volatility of certain comparable public companies, using daily log-returns of stock prices, as adjusted for the impact of financial leverage. The risk-free interest rate reflects the U.S. Treasury yield curve in effect at the time of the grant. The following table summarizes the weighted-average grant-date fair value and assumptions used to develop the fair value estimates for the options granted in 2022. No stock options were granted under the 2021 Plan during the year ended December 31, 2023: 2022 Weighted-average grant-date fair value of options $48.79 Risk-free interest rate 1.74% Term (years) 6.2 Volatility 61.19% Expected dividend —% As of December 31, 2023, the unrecognized compensation expense related to stock options was $2.4 million, to be recognized over a weighted-average period of 1.9 years. Restricted stock units The Company grants restricted stock units (“RSUs”) to members of management, key employees and independent directors. RSUs are time-based vesting awards and are subject to the continued service of the employee or non-employee director over the vesting period. RSUs typically vest in equal annual installments over one Pursuant to the 2021 Plan, total share-based compensation expense related to RSUs recognized in the years ended December 31, 2023 and 2022 was approximately $6.2 million and $4.4 million, respectively. The following table summarizes the activity of unvested RSUs and the respective weighted-average grant date fair value per RSU for the year ended December 31, 2023: 2023 RSUs Weighted-Average Grant Date Fair Value Outstanding and unvested, beginning of year 84,283 $ 99.46 Granted 762,201 16.58 Vested (39,145) 101.33 Forfeited (39,975) 30.22 Outstanding and unvested, end of year 767,364 $ 20.56 During the year ended December 31, 2022, the Company granted approximately 0.1 million RSUs with a weighted-average grant date fair value of $104.51. During the years ended December 31, 2023 and 2022, the fair value of vested RSUs was $4.0 million and $1.8 million, respectively. As of December 31, 2023, the unrecognized compensation expense related to RSUs was $10.9 million, to be recognized over a weighted-average period of 2.0 years. Restricted stock awards The Company grants restricted stock awards (“RSAs”) to members of management and key employees. RSAs are time-based vesting awards and are subject to the continued service of the employee over the vesting period. RSAs typically vest in equal quarterly installments over a service period of three years from the grant date. The fair value of restricted stock was based on the price of the Company’s common stock on the grant date. Pursuant to the 2021 Plan, total share-based compensation expense related to RSAs recognized in the years ended December 31, 2023 and 2022 was approximately $0.4 million and $0.5 million, respectively. The following table summarizes the activity of unvested RSAs and respective weighted-average grant date fair value per RSA for the year ended December 31, 2023: 2023 RSAs Weighted-Average Grant Date Fair Value Outstanding and unvested, beginning of year 3,391 $ 171.00 Granted — — Vested (2,233) 171.00 Forfeited (83) 171.00 Outstanding and unvested, end of year 1,075 $ 171.00 No RSAs were granted during the years ended December 31, 2023 and 2022. During the years ended December 31, 2023 and 2022, the fair value of vested RSAs was $0.4 million and $0.5 million, respectively. As of December 31, 2023, the unrecognized compensation expense related to RSAs was $0.2 million, to be recognized over a weighted-average period of 0.5 years.

Mezzanine and Stockholders' Equ

Mezzanine and Stockholders' Equity	12 Months Ended
Mezzanine and Stockholders' Equity	Dec. 31, 2023
Equity [Abstract]
Mezzanine and Stockholders' Equity	Mezzanine and Stockholders' Equity ATI Physical Therapy, Inc. Series A Senior Preferred Stock In connection with the 2022 Debt Refinancing, the Company issued 165,000 shares of non-convertible preferred stock (the "Series A Senior Preferred Stock") plus warrants to purchase 0.1 million shares of the Company's common stock at an exercise price of $150.00 per share (the "Series I Warrants") and warrants to purchase 0.1 million shares of the Company's common stock at an exercise price equal to $0.50 per share (the "Series II Warrants"). The shares of the Series A Senior Preferred Stock have a par value of $0.0001 per share and an initial stated value of $1,000 per share, for an aggregate initial stated value of $165.0 million. The Company is authorized to issue 1.0 million shares of preferred stock per the Certificate of Designation. As of December 31, 2023, there was 0.2 million shares of Series A Senior Preferred Stock issued and outstanding. The gross proceeds received from the issuance of the Series A Senior Preferred Stock and the Series I and Series II Warrants were $165.0 million during the year ended December 31, 2022, which was allocated among the instruments based on the relative fair values of each instrument. Of the gross proceeds, $144.7 million was allocated to the Series A Senior Preferred Stock, $5.1 million to the Series I Warrants and $15.2 million to the Series II Warrants. The resulting discount on the Series A Senior Preferred Stock will be recognized as a deemed dividend when those shares are subsequently remeasured upon becoming redeemable or probable of becoming redeemable. The Company recognized $2.9 million in issuance costs and $1.4 million of original issue discount related to the Series A Senior Preferred Stock. The following table reflects the components of the initial proceeds related to the Series A Senior Preferred Stock (in thousands): Gross proceeds allocated to Series A Senior Preferred Stock $ 144,667 Less: original issue discount (1,447) Less: issuance costs (2,880) Net proceeds received from issuance of Series A Senior Preferred Stock $ 140,340 The Series A Senior Preferred Stock has priority over the Company's Class A common stock and all other junior equity securities of the Company, and is junior to the Company's existing or future indebtedness and other liabilities (including trade payables), with respect to payment of dividends, distribution of assets, and all other liquidation, winding up, dissolution, dividend and redemption rights. The Series A Senior Preferred Stock carries an initial dividend rate of 12.0% per annum (the "Base Dividend Rate"), payable quarterly in arrears. Dividends will be paid-in-kind and added to the stated value of the Series A Senior Preferred Stock. The Company may elect to pay dividends on the Series A Senior Preferred Stock in cash beginning on the third anniversary of the Refinancing Date and, with respect to any such dividends paid in cash, the dividend rate then in effect will be decreased by 1.0%. The Base Dividend Rate is subject to certain adjustments, including an increase of 1.0% per annum on the first day following the fifth anniversary of the Refinancing Date and on each one-year anniversary thereafter, and 2.0% per annum upon the occurrence of either an Event of Noncompliance (as defined in the Certificate of Designation) or a failure by the Company to redeem in full all Series A Senior Preferred Stock upon a Mandatory Redemption Event, which includes a change of control, liquidation, bankruptcy or certain restructurings. The paid-in-kind dividends related to the Series A Senior Preferred Stock were $23.2 million and $17.9 million for the year ended December 31, 2023 and 2022, respectively. As of December 31, 2023, the accumulated paid-in-kind dividends related to the Series A Senior Preferred Stock were $41.1 million and the aggregate stated value was $206.1 million. Changes in the aggregate stated value and stated value per share of the Series A Senior Preferred Stock consisted of the following (in thousands, except per share data): December 31, 2023 December 31, 2022 Aggregate stated value, beginning of period $ 182,876 $ 165,000 Paid-in-kind dividends (1) 23,219 17,876 Aggregate stated value, end of period $ 206,095 $ 182,876 Preferred shares issued and outstanding, end of period 165 165 Stated value per share, end of period $ 1,249.06 $ 1,108.34 (1) Changes in the stated value for the year ended December 31, 2022 represent changes since the Refinancing Date, which is when the Series A Senior Preferred Stock was issued and established. The Company has the right to redeem the Series A Senior Preferred Stock, in whole or in part, at any time (subject to certain limitations on partial redemptions). The Redemption Price for each share of Series A Senior Preferred Stock is equal to the stated value subject to certain price adjustments depending on when such optional redemption takes place, if at all. The Series A Senior Preferred Stock is perpetual and is not mandatorily redeemable at the option of the holders, except upon the occurrence of a Mandatory Redemption Event. Upon the occurrence of a Mandatory Redemption Event, to the extent not prohibited by law, the Company is required to redeem all Series A Senior Preferred Stock, in cash, at a price per share equal to the then applicable Redemption Price. Because the Series A Senior Preferred Stock is mandatorily redeemable contingent on certain events outside the Company’s control, such as a change in control, and since such events are not currently deemed certain to occur, the Series A Senior Preferred Stock is classified as mezzanine equity in the Company's consolidated balance sheets. If an Event of Noncompliance occurs, then the holders of a majority of the then outstanding shares of Series A Senior Preferred Stock (the “Majority Holders”) have the right to demand that the Company engage in a sale/refinancing process to consummate a Forced Transaction. A Forced Transaction includes a refinancing of the Series A Senior Preferred Stock or a sale of the Company. Upon consummation of any Forced Transaction, to the extent not prohibited by law, the Company is required to redeem all Series A Senior Preferred Stock, in cash, at a price per share equal to the then applicable Redemption Price. Holders of shares of Series A Senior Preferred Stock have no voting rights with respect to the Series A Senior Preferred Stock except as set forth in the Certificate of Designation, other documents entered into in connection with the Purchase Agreement and the transactions contemplated thereby, or as otherwise required by law. For so long as any Series A Senior Preferred Stock is outstanding, the Company is prohibited from taking certain actions without the prior consent of the Majority Holders as set forth in the Certificate of Designation which include: issuing equity securities ranking senior to or pari passu with the Series A Senior Preferred Stock, incurring indebtedness or liens, engaging in affiliate transactions, making restricted payments, consummating certain investments or asset dispositions, consummating a change of control transaction unless the Series A Senior Preferred Stock is redeemed in full, altering the Company’s organizational documents, and making material changes to the nature of the Company’s business. As part of the 2022 Debt Refinancing, the Preferred Equityholders, voting as a separate class, had the right to designate and elect one director to serve on the Company’s board of directors until such time after the Refinancing Date that (i) as of any applicable fiscal quarter end, the Company’s trailing 12-month Consolidated Adjusted EBITDA (as defined in the Certificate of Designation) exceeds $100.0 million, or (ii) the Lead Purchaser ceases to hold at least 50.1% of the Series A Senior Preferred Stock held by it as of the Refinancing Date. As part of the 2023 Debt Restructuring, (1) the Preferred Equityholders’ preexisting rights as holders of the Company’s Series A Senior Preferred Stock to designate and elect one director to the Company’s board of directors (the “Board”) was revised to provide that (a) the Preferred Equityholders have the right to appoint three additional directors to the Board (resulting in the right of the Preferred Equityholders to appoint a total of four directors to the Board) until such time after the Closing Date that the Lead Purchaser (as defined in certain of the transaction agreements entered into in connection with the original issuance of the Series A Senior Preferred Stock) ceases to hold at least 50.1% of the Series A Senior Preferred Stock held by it as of the Closing Date, one of whom must be unaffiliated with (and independent of) the Preferred Equityholders and who must meet the definition of “independent” under the listing standards of the New York Stock Exchange ("NYSE"), and by the SEC; and (b) all such designee directors of the Preferred Equityholders will be subject to consideration by the Board (acting in good faith and consistent with their review of other Board candidates) and (2) the provision in the Certificate of Designation of the Company’s Series A Senior Preferred Stock that eliminated the Preferred Equityholders’ director designation rights upon the Company’s achievement of certain amounts of EBITDA was deleted. Prior to the closing of the 2023 Debt Restructuring, because the Series A Senior Preferred Stock is classified as mezzanine equity and was not considered redeemable or probable of becoming redeemable, the paid-in-kind dividends that were added to the stated value did not impact the carrying value of the Series A Senior Preferred Stock in the Company’s consolidated balance sheets. Based on the voting rights associated with the Series B Preferred Stock attached to the 2L Notes issued as part of the 2023 Debt Restructuring, the Company determined that redemption of the Series A Senior Preferred Stock is no longer solely within the control of the Company. As a result, the Company determined that the Series A Senior Preferred Stock is probable of becoming redeemable based on the accounting guidance in ASC Topic 480, Distinguishing Liabilities from Equity . Following the 2023 Debt Restructuring, since the Series A Senior Preferred Stock is probable of becoming redeemable, the Company will recognize changes in the redemption value of the Series A Senior Preferred Stock immediately as they occur and adjust the carrying amount as if redemption were to occur at the end of the reporting period. As of December 31, 2023, the redemption value of the Series A Senior Preferred Stock was $220.4 million, which includes the aggregate stated value at December 31, 2023, inclusive of paid-in-kind dividends, and an incremental redemption value adjustment to reflect the carrying amount equal to what the redemption amount would be as if redemption were to occur at the end of the reporting period, based on the terms of the Certificate of Designation. Changes in the carrying value of the Series A Senior Preferred Stock consisted of the following for the year ended December 31, 2023 (in thousands). There were no changes in carrying value in 2022. December 31, 2023 Carrying value, beginning of period $ 140,340 Write off original issue discount 1,447 Write off issuance costs 2,880 Deemed dividend from discount on initial gross proceeds allocation 20,333 Paid-in-kind dividends recognized to carrying value 41,095 Redemption value adjustment 14,298 Carrying value, end of period $ 220,393 2022 Warrants In connection with the Preferred Stock Financing, the Company agreed to issue to the preferred stockholders the Series I Warrants entitling the holders thereof to purchase 0.1 million shares of the Company's common stock at an exercise price equal to $150.00 per share, exercisable for 5 years from the Refinancing Date; and the Series II Warrants entitling holders thereof to purchase 0.1 million shares of the Company's common stock at an exercise price equal to $0.50 per share, exercisable for 5 years from the Refinancing Date (collectively, the "2022 Warrants"). Such number of shares of common stock purchasable pursuant to the 2022 Warrant Agreement and related exercise prices may be adjusted from time to time under certain scenarios as set forth in the 2022 Warrant Agreement, which relate to potential changes in the Company's capital structure. The 2022 Warrants are classified as equity instruments and were initially recorded at an amount equal to the proceeds received from the Preferred Stock Financing allocated among the Series A Senior Preferred Stock, the Series I Warrants, and the Series II Warrants based upon their relative fair values. Of the gross proceeds, $5.1 million was allocated to the Series I Warrants and $15.2 million was allocated to the Series II Warrants. The Company recognized total issuance costs and original issue discount of approximately $0.2 million and $0.5 million related to the Series I Warrants and Series II Warrants, respectively, during the year ended December 31, 2022. The following table reflects the components of proceeds related to the 2022 Warrants (in thousands): Series I Warrants Series II Warrants Total Gross proceeds allocated to 2022 Warrants $ 5,101 $ 15,232 $ 20,333 Less: original issue discount (51) (152) (203) Less: issuance costs (102) (303) (405) Net proceeds received from issuance of 2022 Warrants $ 4,948 $ 14,777 $ 19,725 Class A common stock The Company is authorized to issue 470.0 million shares of Class A common stock with a par value of $0.0001 per share. Holders of the Company’s Class A common stock are entitled to one vote for each share on each matter on which they are entitled to vote. At December 31, 2023, there were 4.2 million shares of Class A common stock issued and 4.0 million shares outstanding. As of December 31, 2023, shares of Class A common stock reserved for potential future issuance, on an as-if converted basis, were as follows (in thousands): December 31, 2023 2L Notes (1) 8,625 Shares available for grant under the 2021 Plan 241 2021 Plan share-based awards outstanding 868 Earnout Shares reserved 300 2022 Warrant shares reserved 230 IPO Warrant shares reserved 197 Vesting Shares reserved (2) 173 Restricted shares (2) 5 Total shares of common stock reserved 10,639 (1) Calculated based on the principal amount of 2L Notes and Conversion Price of $12.50 per share. This figure differs from the contractual Voting Rights Conversion Price of $12.87 as outlined in Note 8 - Borrowings. (2) Represents shares of Class A common stock legally issued, but not outstanding, as of December 31, 2023. Treasury stock During the year ended December 31, 2023, the Company net settled 5,254 shares of its Class A common stock related to employee tax withholding obligations associated with the Company's share-based compensation program. These shares are reflected at cost as treasury stock in the consolidated financial statements. As of December 31, 2023, there were 6,794 shares of treasury stock totaling $0.2 million recognized in the consolidated balance sheets.

IPO Warrant Liability

IPO Warrant Liability	12 Months Ended
IPO Warrant Liability	Dec. 31, 2023
Other Liabilities Disclosure [Abstract]
IPO Warrant Liability	IPO Warrant Liability The Company has outstanding public warrants to purchase an aggregate of approximately 0.1 million shares of the Company’s Class A common stock at an exercise price of $575.00 per share ("Public Warrants") and outstanding private placement warrants to purchase an aggregate of approximately 0.1 million shares of the Company's Class A common stock at an exercise price of $575.00 per share ("Private Placement Warrants") (collectively, the "IPO Warrants"). As of December 31, 2023, the Public Warrants remain delisted from the NYSE and are traded in the over-the-counter market. There were no IPO Warrants exercised during the year ended December 31, 2023. The Company accounts for its outstanding IPO Warrants in accordance with the guidance contained in ASC Topic 815-40, Derivatives and Hedging - Contracts on an Entity’s Own Equity, and determined that the IPO Warrants do not meet the criteria for equity treatment thereunder. As such, each IPO Warrant must be recorded as a liability and is subject to re-measurement at each balance sheet date. Refer to Note 14 - Fair Value Measurements for further details. Changes in fair value are recognized in change in fair value of warrant liability in the Company’s consolidated statements of operations. The following table presents the change in the fair value of Private Placement Warrants that is recognized in change in fair value of warrant liability in the consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Fair value, beginning of period $ 29 $ 1,305 Decrease in fair value (28) (1,276) Fair value, end of period $ 1 $ 29 The following table presents the changes in the fair value of the Public Warrants that is recognized in change in fair value of warrant liability in the consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Fair value, beginning of period $ 69 $ 3,036 Decrease in fair value (67) (2,967) Fair value, end of period $ 2 $ 69 Each Public Warrant entitles the holder to purchase one share of Class A common stock at an exercise price of $575.00 per share, subject to adjustment. The Public Warrants became exercisable 30 days after the completion of the Business Combination, subject to certain conditions, including that the Company maintains an effective registration statement under the Securities Act covering the Class A common stock issuable upon exercise of the Public Warrants. The Public Warrants will expire five years after the completion of the Business Combination or earlier upon redemption or liquidation. The Company may call the Public Warrants for redemption for cash or for Class A common stock under certain circumstances. The Private Placement Warrants are identical to the Public Warrants, except that (i) the Private Placement Warrants and the Class A common stock issuable upon exercise of the Private Placement Warrants were not transferable, assignable or salable until 30 days after the completion of the Business Combination, subject to certain limited exceptions, (ii) the Private Placement Warrants are non-redeemable (except under certain circumstances) so long as they are held by the initial purchasers or such purchasers’ permitted transferees, (iii) the Private Placement Warrants may be exercised by the holders on a cashless basis, and (iv) the Private Placement Warrants and the Class A common stock issuable upon exercise of the Private Placement Warrants are entitled to registration rights. If the Private Placement Warrants are held by someone other than the initial stockholders or their permitted transferees, the Private Placement Warrants will be redeemable by the Company in all redemption scenarios and exercisable by such holders on the same basis as the Public Warrants. The exercise price and number of Class A common stock issuable upon exercise of the IPO Warrants may be adjusted in certain circumstances including in the event of a stock dividend, recapitalization, reorganization, merger or consolidation.

Contingent Common Shares Liabil

Contingent Common Shares Liability	12 Months Ended
Contingent Common Shares Liability	Dec. 31, 2023
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Contingent Common Shares Liability	Contingent Common Shares Liability Earnout Shares Subject to the terms and conditions of the merger agreement between Wilco Holdco, Inc. and FAII, certain stockholders of Wilco Holdco, Inc. were provided the contingent right to receive, in the aggregate, up to 0.3 million shares of Class A common stock if, from the closing of the Company's business combination with FAII until the 10 th anniversary thereof, the dollar volume-weighted average price (“VWAP”) of Class A common stock exceeds certain thresholds (the "Earnout Shares"). The Earnout Shares vest in three equal and separate tranches of 0.1 million shares each if the VWAP of Class A common stock exceeds $600.00, $700.00 and $800.00 per share, respectively, for any 5 trading days within any consecutive 10 trading day period. The Earnout Shares are subject to acceleration in the event of a sale or other change in control if the holders of Class A common stock would receive a per share price in excess of the applicable Earnout Shares price target. The Company accounts for the potential Earnout Shares as a liability in accordance with the guidance in ASC Topic 480, Distinguishing Liabilities from Equity , and ASC Topic 815, Derivatives and Hedging, and is subject to re-measurement at each balance sheet date. Changes in fair value are recognized in change in fair value of contingent common shares liability in the Company’s consolidated statements of operations. As of December 31, 2023, no Earnout Shares have been issued as none of the corresponding share price thresholds have been met. The following table presents the changes in the fair value of the Earnout Shares that is recognized in change in fair value of contingent common shares liability in the consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Fair value, beginning of period $ 1,800 $ 28,800 Decrease in fair value (1,433) (27,000) Fair value, end of period $ 367 $ 1,800 Refer to Note 14 - Fair Value Measurements for further details. Vesting Shares Subject to the terms and conditions of the sponsor letter agreement that was executed in connection with the merger agreement between Wilco Holdco, Inc. and FAII, approximately 0.2 million shares of Class F common stock of FAII outstanding immediately prior to the Company's business combination with FAII converted to potential Class A common shares and became subject to vesting and forfeiture provisions (the "Vesting Shares"). The Vesting Shares vest in three equal and separate tranches of approximately 0.1 million shares each if the VWAP of Class A common stock exceeds $600.00, $700.00 and $800.00 p er share, respectively, for any 5 trading days within any consecutive 10 trading day period. The Vesting Shares are subject to acceleration in the event of a sale or other change in control if the holders of Class A common stock would receive a per share price in excess of the applicable Vesting Shares price target. The Company accounts for the Vesting Shares as a liability in accordance with the guidance in ASC Topic 480, Distinguishing Liabilities from Equity , and ASC Topic 815, Derivatives and Hedging, and is subject to re-measurement at each balance sheet date. Changes in fair value are recognized in change in fair value of contingent common shares liability in the Company’s consolidated statements of operations. As of December 31, 2023, no Vesting Shares are outstanding as none of the corresponding share price thresholds have been met. The following table presents the changes in the fair value of the Vesting Shares that is recognized in change in fair value of contingent common shares liability in the consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Fair value, beginning of period $ 1,035 $ 16,560 Decrease in fair value (824) (15,525) Fair value, end of period $ 211 $ 1,035 Refer to Note 14 - Fair Value Measurements for further details.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
Fair Value Measurements	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The Company determines fair value measurements used in its consolidated financial statements based upon the price that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Fair value is estimated by applying the following hierarchy, which prioritizes the inputs used to measure fair value into three levels, with Level 1 having the highest priority and Level 3 having the lowest. • Level 1: Observable inputs, which include unadjusted quoted prices in active markets for identical instruments. • Level 2: Observable inputs other than Level 1 inputs, such as quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the instruments. • Level 3: Unobservable inputs in which little or no market data exists, therefore requiring an entity to develop its own assumptions, such as valuations derived from valuation techniques in which one or more significant inputs or significant value drivers are unobservable. As of December 31, 2023 and December 31, 2022 , respectively, the recorded values of cash, cash equivalents and restricted cash, accounts receivable, other current assets, accounts payable, accrued expenses and deferred revenue approximate their fair values due to the short-term nature of these items. Money market funds categorized in Level 1 of the fair value hierarchy are measured at fair value based on quoted market prices. As of December 31, 2023 and December 31, 2022, respectively, the fair value of money market fund investments i ncluded in cash and cash equivalents w as zero and $30.0 million . Fair value measurement of debt The Company's Revolving Loans are Level 2 fair value measures which have a variable interest rate structure that resets on a frequent short-term basis and, as of December 31, 2023, the recorded amounts approximate fair value. Prior to the 2023 Debt Restructuring, the Company's Senior Secured Term Loan was a Level 2 fair value measure. The Company utilized market approach valuation techniques based on interest rates and credit data that are currently available to the Company for issuance of debt with similar terms or maturities. In connection with the 2023 Debt Restructuring, the Company estimated the fair value of a portion of its Senior Secured Term Loan using a Black-Derman-Toy Lattice Bond Pricing Model, which utilized Level 3 inputs. During the third quarter of 2023, the Company prospectively changed its method to estimate the fair value of its Senior Secured Term Loan to a Discounted Cash Flow Model, noting no material changes to the presentation of fair values relative to the previous method. The Discounted Cash Flow Model utilizes observable and unobservable Level 3 inputs, such as SOFR forward rates and an estimated yield. As of December 31, 2023, the carrying amount and estimated fair value of the Senior Secured Term Loan was approximately $395.1 million and $369.0 million, respectively. As discussed in Note 8 - Borrowings , the Company has made an irrevocable election to account for the 2L Notes under the fair value option in accordance with ASC Topic 825, Financial Instruments . As such, the 2L Notes are initially recorded as a liability at estimated fair value and are subject to re-measurement at each balance sheet date with changes in fair value recognized in the Company's consolidated statements of operations. The Company determines the fair value of the 2L Notes using Level 3 inputs. In connection with the 2023 Debt Restructuring, the fair value of the 2L Notes was estimated using a Goldman Sachs Convertible Bond Valuation Model to consider the impacts of the conversion feature. During the third quarter of 2023, the Company prospectively changed its method to estimate the fair value of its 2L Notes to a Bond Plus Call Model, which also considers the impacts of the conversion feature, noting no material changes to the presentation of fair values relative to the previous method. Changes in the assumptions of the unobservable inputs may materially affect the estimated fair value of the 2L Notes. The key inputs into the respective valuation models used to estimate the fair value of the 2L Notes were as follows as of December 31, 2023 and the Closing Date, which is when the 2L Notes were issued: 2L Notes December 31, 2023 June 15, 2023 Risk-free interest rate 3.83% 3.90% Volatility 45.00% 50.00% Selected yield 20.50% 20.00% Expected term (years) 4.7 5.3 Share price $6.14 $10.21 The following table presents the changes in the fair value of the 2L Notes that is recognized in change in fair value of 2L Note s in the consolidated statements of operations for the periods indicated below (in thousands). None of the change in fair value is attributable to instrument-specific credit risk: Year Ended December 31, 2023 Fair value, beginning of period (1) $ 103,943 Decrease in fair value (1) (24,471) Fair value, end of period $ 79,472 (1) Represents changes in fair value from the Closing Date, which is when the 2L Notes were issued. Fair value measurement of share-based financial liabilities Prior to June 30, 2023, the Company determined the fair value of the Public Warrant liability using Level 1 inputs, and determined the fair value of the Private Placement Warrant liability using the price of the Public Warrants as a Level 2 input. Beginning June 30, 2023, the Company determined the fair value of the IPO Warrant liability using Level 3 inputs as its Public Warrants were delisted from the NYSE. As of December 31, 2023, t he Company determined the fair value of the IPO Warrant liability, Earnout Shares liability and Vesting Shares liability using Level 3 inpu ts. The warrants would be deemed exercisable or redeemable if the Company's common stock price over a specified measurement period was trading at certain thresholds. The contingent common shares contain specific market conditions to determine whether the shares vest based on the Company’s common stock price over a specified measurement period. Given the path-dependent nature of the requirement in which the warrants are exercised or redeemed, and the shares are earned, a Monte-Carlo simulation was used to estimate the fair value of the liabilities. The Company’s common stock price was simulated to each measurement period based on the above methodology. In each iteration, the simulated stock price was compared to the conditions under which the warrants are exercised or redeemed, or the shares vest. In iterations where the stock price corresponded to warrants being exercised or redeemed, or shares vesting, the future value of the warrants or contingent common shares were discounted back to present value. The fair value of the liabilities were estimated based on the average of all iterations of the simulation. Inherent in a Monte-Carlo valuation model are assumptions related to expected stock-price volatility, expected term, risk-free interest rate and dividend yield. The Company estimates the volatility based on the historical volatility of certain guideline companies, as well as the Company's historical volatility over the available look-back period as of the valuation date. The risk-free interest rate is based on the U.S. Treasury zero-coupon yield curve on the valuation date for a maturity similar to the expected term of the IPO Warrants, Earnout Shares and Vesting Shares. The dividend yield percentage is zero based on the Company's current expectations related to the payment of dividends during the expected term of the IPO Warrants, Earnout Shares or Vesting Shares. The key inputs into the Monte-Carlo option pricing model were as follows as of December 31, 2023 and December 31, 2022 for the respective Level 3 instruments: IPO Warrants Earnout Shares and Vesting Shares December 31, 2023 December 31, 2022 December 31, 2023 December 31, 2022 Risk-free interest rate 4.09% N/A 3.84% 3.88% Volatility 98.10% N/A 77.20% 74.60% Dividend yield —% N/A —% —% Expected term (years) 2.5 N/A 7.5 8.5 Share price $6.14 N/A $6.14 $15.50 Refer to Note 12 - IPO Warrant Liability and Note 13 - Contingent Common Shares Liability for further details on the change in fair value of the IPO Warrants and change in fair value of the Earnout Shares and Vesting Shares, respectively. Fair value measurement of interest rate derivative instruments The Company is exposed to interest rate variability with regard to its existing variable-rate debt instrument, which exposure primarily relates to movements in various interest rates, such as SOFR. The Company utilizes interest rate cap derivative instruments for purposes of hedging exposures related to such variable-rate cash payments. The Company's interest rate caps have historically been designated as cash flow hedging instruments. During the third quarter of 2023, the Company made a 12-month SOFR election on its Senior Secured Term Loan and, as a result, the Company's interest rate cap no longer qualifies as a designated cash flow hedging instrument. The Company records derivatives on the balance sheet at fair value, which represents the estimated amounts it would receive or pay upon termination of the derivative prior to the scheduled expiration date. The fair value is derived from model-driven information based on observable Level 2 inputs, such as SOFR forward rates. For derivatives designated and that qualify as a cash flow hedge of interest rate risk, the gain or loss on the derivative is recorded in accumulated other comprehensive income and subsequently reclassified into interest expense in the same period(s) during which the hedged transaction affects earnings. For derivatives that are considered to be ineffective, or are not designated in a hedging relationship, the gain or loss on the derivative is immediately recognized in other expense (income), net. The following table presents the activity of cash flow hedges included in accumulated other comprehensive income (loss) for the years ended December 31, 2023 and 2022, respectively (in thousands): Cash Flow Hedges Balance as of December 31, 2022 $ 4,899 Unrealized gain recognized in other comprehensive income before reclassifications 801 Reclassification to interest expense, net (5,294) Balance as of December 31, 2023 $ 406 Balance as of December 31, 2021 $ 28 Unrealized gain recognized in other comprehensive income before reclassifications 8,310 Reclassification to interest expense, net (3,439) Balance as of December 31, 2022 $ 4,899 For the year ended December 31, 2023, the change in fair value of the Company's non-designated cash flow hedge was immaterial. The following table presents t he fair value of derivative assets and liabilities within the consolidated balance sheets as of December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Assets Liabilities Assets Liabilities Derivatives not designated as cash flow hedging instruments: Other current assets $ 33 — — — Other non-current assets — — — — Accrued expenses and other liabilities — — — — Other non-current liabilities — $ 62 — — Derivatives designated as cash flow hedging instruments: Other current assets — — $ 5,028 — Other non-current assets — — — — Accrued expenses and other liabilities — — — — Other non-current liabilities — — — $ 73

Income Taxes

Income Taxes	12 Months Ended
Income Taxes	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company's (loss) income before taxes consists of only domestic operations. The details of the Company's income tax expense (benefit) for the years ended December 31, 2023 and 2022 are as follows (in thousands): 2023 2022 Current: Federal $ — $ 6 State 87 37 Total current 87 43 Deferred: Federal 745 (37,634) State 1,736 (10,939) Total deferred 2,481 (48,573) Total income tax expense (benefit) $ 2,568 $ (48,530) The effective tax rate for the years ended December 31, 2023 and 2022 was (4.0)% and 9.0%, respectively. The Company's effective income tax rate varies from the federal statutory rate due to various items, such as state income taxes, valuation allowances and nondeductible items such as fair value adjustments related to liability-classified share-based instruments and impairment charges. The differences between the federal tax rate and the Company's effective tax rate for the years ended December 31, 2023 and 2022 are as follows (in thousands): 2023 2022 Federal income tax benefit at statutory rate $ (13,337) 21.0 % $ (113,731) 21.0 % State income tax benefit, net of federal tax benefit (5,847) 9.2 % (16,827) 3.1 % Change in state tax rate (576) 0.9 % 5 — % Share-based compensation 1,410 (2.2) % (304) 0.1 % Prior period adjustments and other 1,685 (2.7) % 471 (0.1) % Valuation allowance 20,235 (31.9) % 31,595 (5.8) % Changes in fair value of warrant liability and contingent common shares liability (494) 0.8 % (9,821) 1.8 % Goodwill impairment charges — — % 59,893 (11.1) % Other permanent differences, net (508) 0.9 % 189 — % Total income tax expense (benefit) $ 2,568 (4.0) % $ (48,530) 9.0 % Deferred income taxes have been provided on temporary differences, which consist of the following at December 31, 2023 and 2022 (in thousands): 2023 2022 Deferred income tax assets: Accrued liabilities $ 8,895 $ 7,112 Provision for bad debt 12,097 11,828 Operating lease liabilities 56,661 64,288 Acquisition and transaction costs 2,699 3,186 Net operating losses 116,834 104,419 Interest expense 54,796 43,323 Other deferred tax assets 6,429 6,335 Total gross deferred income tax assets 258,411 240,491 Valuation allowance (110,142) (89,907) Total gross deferred income tax assets, net of valuation allowance 148,269 150,584 Deferred income tax liabilities: Goodwill 32,498 26,251 Trade name/trademark 66,183 66,445 Operating lease right-of-use assets 46,106 54,360 Depreciation 14,158 20,039 Deferred debt issuance costs and original issue discount 8,708 — Other deferred tax liabilities 1,983 2,375 Total gross deferred income tax liabilities 169,636 169,470 Net deferred income tax liabilities $ 21,367 $ 18,886 Deferred tax assets include federal net operating losses of $77.8 million and $68.9 million at December 31, 2023 and 2022, respectively, and state net operating losses of $39.1 million and $35.5 million at December 31, 2023 and 2022, respectively. Deferred tax assets are expected to be used in the reduction of taxable earnings of future tax years unless it is determined they are not more likely than not to be realized based on the weight of available evidence. The earliest net operating loss will expire by statute in 2024 for state net operating losses, and in 2036 for federal net operating losses. In evaluating the Company's ability to recover deferred income tax assets, all available positive and negative evidence is considered, including scheduled reversal of deferred tax liabilities, operating results and forecasts of future taxable income in each of the jurisdictions in which the Company operates. As of December 31, 2023, the Company determined that a significant portion of its federal and state net operating loss carryforwards with definite and certain indefinite carryforward periods and certain deferred tax assets were not more likely than not to be realized based on the weight of available evidence. As a result, the Company recorded an increase of $13.4 million to its valuation allowance related to federal net operating loss and interest expense carryforwards and an increase of $6.8 million to its valuation allowance related to state net operating loss carryforwards and certain deferred tax assets. These amounts were recorded during the year ended December 31, 2023 in income tax expense (benefit) in the consolidated statements of operations. As of December 31, 2022, the Company determined that a significant portion of its federal and state net operating loss carryforwards with definite and certain indefinite carryforward periods and certain deferred tax assets were not more likely than not to be realized based on the weight of available evidence. As a result, the Company recorded an increase of $25.8 million to its valuation allowance related to federal net operating loss and interest expense carryforwards and an increase of $5.8 million to its valuation allowance related to state net operating loss carryforwards and certain deferred tax assets. These amounts were recorded during the year ended December 31, 2022 in income tax expense (benefit) in the consolidated statements of operations. On June 15, 2023, the Company experienced an ownership change for purposes of Section 382 of the Internal Revenue Code of 1986, as amended. The net operating losses and interest expense carryovers in existence as of the date of the ownership change remain available to offset future taxable income during the carryforward periods based on limitations under Section 382. The Company is routinely audited by the tax authorities in various U.S. states and is currently not subject to examination. The statute remains open for most state jurisdictions for periods beginning in 2019. For federal tax purposes, tax years through 2019 are closed for examination by the Internal Revenue Service. Any interest and penalties related to the tax uncertainties are recorded in income tax (benefit) expense.

Leases

Leases	12 Months Ended
Leases	Dec. 31, 2023
Leases [Abstract]
Leases	Leases The Company leases various facilities and office equipment for its physical therapy operations and administrative support functions under operating leases. The Company’s initial operating lease terms are generally between 7 and 10 years, and typically contain options to renew for varying terms. Right-of-use ("ROU") assets and lease liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. The amortization of operating lease ROU assets and the accretion of operating lease liabilities are reported together as fixed lease expense. The fixed lease expense is recognized on a straight-line basis over the life of the lease. If the ROU asset has been impaired, lease expense is no longer recognized on a straight-line basis. The lease liability continues to amortize using the effective interest method, while the ROU asset is subsequently amortized on a straight-line basis. Refer to Note 2 - Basis of Presentation and Summary of Significant Accounting Policies for more information about the Company's lease accounting policies. Lease costs are included as components of rent, clinic supplies, contract labor and other and selling, general and administrative expenses on the consolidated statements of operations. Lease charges related to ROU asset impairments are included in goodwill, intangible and other asset impairment charges on the consolidated statements of operations. The components of the Company's lease costs incurred were as follows for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Lease cost Operating lease cost (1) $ 68,924 $ 69,533 Variable lease cost (2) 22,086 20,951 Total lease cost (3) $ 91,010 $ 90,484 (1) Includes ROU asset impairment charges of $2.3 million and $2.6 million for the years ended December 31, 2023 and 2022, respectively. (2) Includes short term lease costs, which are immaterial. (3) Sublease income primarily relates to subleases of certain clinic facilities to third parties, and is immaterial. The Company leases its executive offices under an operating lease expiring in December 2032. In December 2023, the Company entered into an agreement to sublease a portion of the office space effective on January 1, 2024 and the entire office space effective on January 1, 2025. The Company recognized initial broker commissions costs related to executing the sublease in other non-current assets and accrued expenses and other liabilities in the Company's consolidated balance sheets, which are immaterial. The costs will amortize ratably over the sublease term in selling, general and administrative expenses on the consolidated statements of operations. During the years ended December 31, 2023 and 2022, the Company modified the lease terms for a significant number of its real estate leases, primarily related to lease term extensions and renewals in the normal course of business. Modifications during the years ended December 31, 2023 and 2022 contributed an increase to the Company’s operating lease ROU assets and operating lease liabilities of approximately $12.1 million and $13.7 million, respectively. Other supplemental quantitative disclosures were as follows for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 65,870 $ 72,440 Right-of-use assets obtained in exchange for new operating lease liabilities $ 7,293 $ 9,688 Average lease terms and discount rates as of December 31, 2023 and December 31, 2022 were as follows: December 31, 2023 December 31, 2022 Weighted-average remaining lease term: Operating leases 5.4 years 5.9 years Weighted-average discount rate: Operating leases 7.4% 6.9% Estimated undiscounted future lease payments under non-cancellable operating leases, along with a reconciliation of the undiscounted cash flows to operating lease liabilities, respectively, at December 31, 2023 were as follows (in thousands): Year Amount (1) 2024 $ 66,641 2025 56,874 2026 49,925 2027 38,606 2028 27,185 Thereafter 50,955 Total undiscounted future cash flows 290,186 Less: Imputed Interest (53,054) Present value of future cash flows $ 237,132 Presentation on Balance Sheet: Current $ 51,530 Non-current $ 185,602 (1) Excludes $0.4 million of current portion of operating lease liabilities and $1.4 million of operating lease liabilities, respectively, reclassified as held for sale as of December 31, 2023. Refer to Note 3 - Divestitures for additional information.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies The Company has contractual commitments that are not required to be recognized in the consolidated financial statements related to cloud computing and telecommunication services agreements. As of December 31, 2023, minimum amounts due under these agreements are approximately $11.6 million through January of 2026 subject to customary business terms and conditions. On January 16, 2024, the Company entered into a networking technology and telecommunications service agreement with a contractual commitment that is not required to be recognized in the consolidated financial statements. The minimum amount due under this agreement is approximately $27.0 million through May 2029, subject to customary business terms and conditions. From time to time, the Company is a party to legal proceedings, governmental audits and investigations that arise in the ordinary course of business. Management is not aware of any legal proceedings, governmental audits and investigations of which the outcome is probable to have a material adverse effect on the Company’s results of operations, cash flows or financial condition. The outcome of any litigation and claims against the Company cannot be predicted with certainty, and the resolution of current or future claims could materially affect our future results of operations, cash flows or financial condition. The Company recognizes loss contingencies related to legal matters when a loss is both probable and reasonably estimable, and provides disclosures for loss contingencies that do not meet both of these conditions if there is a reasonable possibility that a loss has been incurred. Legal fees are expensed as incurred. During 2022, the Company engaged in discussions with a payor regarding a billing dispute related to certain historical claims. Management believed, based on discussions with its legal counsel, that the Company had meritorious defenses against such unasserted claim. However, based on the progress of settlement discussions to avoid the cost of potential litigation, the Company recorded a charge for a net settlement liability related to the billing dispute of $3.0 million, which is included in selling, general and administrative expenses in its consolidated statements of operations for the year ended December 31, 2022. As of December 31, 2022, the liability was fully settled. Stockholder class action complaints Federal Securities Litigation. On August 16, 2021, two purported ATI stockholders, Kevin Burbige and Ziyang Nie, filed a putative class action complaint in the U.S. District Court for the Northern District of Illinois against ATI, Labeed Diab, Joe Jordan, and Drew McKnight (collectively, the “ATI Individual Defendants”), and Joshua Pack, Marc Furstein, Leslee Cowen, Aaron Hood, Carmen Policy, Rakefet Russak-Aminoach, and Sunil Gulati (collectively, the “FVAC Defendants”). On October 7, 2021, another purported ATI stockholder, City of Melbourne Firefighters' Retirement System ("City of Melbourne"), filed a nearly identical putative class action complaint in the U.S. District Court for the Northern District of Illinois against ATI, the ATI Individual Defendants, and the FVAC Defendants. On November 18, 2021, the court consolidated the cases and appointed The Phoenix Insurance Company Ltd. and The Phoenix Pension & Provident Funds as lead plaintiffs (together, “Lead Plaintiffs”). On February 8, 2022, Lead Plaintiffs filed a consolidated amended complaint against ATI, the ATI Individual Defendants, and the FVAC Defendants, which asserts claims against (i) ATI and the ATI Individual Defendants under Section 10(b) of the Exchange Act; (ii) the ATI Individual Defendants under Section 20(a) of the Exchange Act (in connection with the Section 10(b) claim); (iii) all defendants under Section 14(a) of the Exchange Act; and (iv) the ATI Individual Defendants and the FVAC Defendants under Section 20(a) of the Exchange Act (in connection with the Section 14(a) claim). Lead Plaintiffs purport to assert these claims on behalf of those ATI stockholders who purchased or otherwise acquired their ATI shares between February 22, 2021 and October 19, 2021, inclusive, and/or held FVAC Class A common shares as of May 24, 2021 and were eligible to vote at FVAC’s June 15, 2021 special meeting. The consolidated amended complaint generally alleges that the proxy materials for the FVAC/ATI merger, as well as other ATI disclosures (including the press release announcing ATI’s financial results for the first quarter of 2021), were false and misleading (and, thus, in violation of Sections 10(b) and 14(a) of the Exchange Act) because they failed to disclose that: (i) ATI was experiencing attrition among its physical therapists; (ii) ATI faced increasing competition for clinicians in the labor market; (iii) as a result, ATI faced difficulty retaining therapists and incurred increased labor costs; (iv) also as a result, ATI would open fewer new clinics; and (v) also as a result, the defendants’ positive statements about ATI’s business, operations, and prospects were materially misleading and/or lacked a reasonable basis. Lead Plaintiffs, on behalf of themselves and the putative class, seek money damages in an unspecified amount and costs and expenses, including attorneys’ and experts’ fees. On April 11, 2022, defendants filed motions to dismiss the consolidated amended complaint, which were fully briefed as of July 25, 2022. On September 6, 2023, the court granted in part and denied in part the motions to dismiss. On October 19, 2023, ATI, the ATI Individual Defendants, and the FVAC Defendants answered the consolidated amended complaint. Discovery then commenced. Thereafter, the parties reached an agreement in principle to resolve all claims in this action for $20.0 million (to be paid entirely by insurance), which agreement remains subject to the negotiation of formal settlement documentation, notice to the putative class, and court approval. The parties have the right to terminate the agreement in principle under certain conditions. The Company recorded an estimated liability of $20.0 million related to this agreement in principle, which is included in accrued expenses and other liabilities in its consolidated balance sheets as of December 31, 2023, and a corresponding insurance recovery receivable of $20.0 million, which is included in insurance recovery receivable in its consolidated balance sheets as of December 31, 2023. Delaware Litigation. On February 7, 2023, another purported ATI stockholder, Wendell Robinson, filed a putative class action complaint in the Court of Chancery of the State of Delaware against Fortress Acquisition Sponsor II, LLC, Andrew A. McKnight, Joshua A. Pack, Marc Furstein, Leslee Cowen, Aaron F. Hood, Carmen A. Policy, Rakefet Russak-Aminoach, Sunil Gulati, Daniel N. Bass, Micah B. Kaplan and Labeed Diab (the "Robinson Action"). The complaint asserts claims against: (i) Fortress Acquisition Sponsor II, LLC, Andrew A. McKnight, Joshua A. Pack, Marc Furstein, Leslee Cowen, Aaron F. Hood, Carmen A. Policy, Rafeket Russak-Aminoach, Sunil Gulati, Daniel N. Bass and Micah B. Kaplan for breach of fiduciary duty; and (ii) Labeed Diab for aiding and abetting breach of fiduciary duty. Plaintiff's allegations generally mirror those asserted in the federal stockholder class action described above, and Plaintiff further alleges that the alleged misrepresentations and omissions in the proxy materials for the FVAC/ATI merger prevented stockholders from making a fully informed decision on whether to approve the merger or have their shares redeemed. Defendants filed motions to dismiss on April 28, 2023, which were fully briefed as of June 23, 2023 and remain pending. On June 1, 2023, another purported ATI stockholder, Phillip Goldstein, filed a putative class action and derivative complaint in the Court of Chancery of the State of Delaware against Labeed Diab, Joseph Jordan, Cedric Coco, Ray Wahl, John L. Larsen, John Maldonado, Carmine Petrone, Joanne M. Burns, Christopher Krubert, James E. Parisi, Joshua A. Pack, Andrew A. McKnight, Marc Furstein, Aaron F. Hood, Carmen A. Policy, Sunil Gulati, Leslee Cowen, and Rakefet Russak-Aminoach (the "Goldstein Action"). The complaint asserts direct and/or derivative claims against: (i) Labeed Diab, Joseph Jordan, Cedric Coco, Ray Wahl, John Larsen, John Maldonado, Carmine Petrone, Joanne Burns, Christopher Krubert, and James Parisi for tortious interference with redemption rights, aiding and abetting breach of fiduciary duty, and fraud; and (ii) Joshua A. Pack, Andrew A. McKnight, Marc Furstein, Aaron F. Hood, Carmen A. Policy, Sunil Gulati, Leslee Cowen, and Rakefet Russak-Aminoach for breach of fiduciary duty. Plaintiff’s allegations generally mirror those asserted in the Robinson Action referenced above. Defendants have not yet responded to the complaint. On August 16, 2023, Plaintiffs in the Robinson and Goldstein Actions filed a motion for consolidation of the Robinson and Goldstein Actions and for appointment of lead plaintiff and lead counsel. On August 31, 2023, defendants opposed the motion for consolidation and concurrently moved to stay the Goldstein Action pending a decision on the motions to dismiss in the Robinson Action. The motion for consolidation and the motion to stay were fully briefed as of September 20, 2023. A hearing was held on October 6, 2023, at which the court (i) denied the motion for consolidation (without prejudice to renewing the motion post-decision on the motions to dismiss in the Robinson Action) and (ii) granted the motion to stay the Goldstein Action (pending the same decision). A hearing on defendants’ motions to dismiss the Robinson Action was held on December 1, 2023, after which the court reserved judgment. The Company has determined that potential liabilities related to the Robinson and Goldstein Actions are not considered probable or reasonably estimable at this time. Stockholder derivative complaint Federal Derivative Litigation. Between December 1, 2021 and September 22, 2022, five purported ATI stockholders filed four derivative actions, purportedly on behalf of ATI, in the U.S. District Court for the Northern District of Illinois. On November 21, 2022, four of these stockholder plaintiffs, Vinay Kumar, Brendan Reginbald, Ziyang Nie and Julia Chang, filed a consolidated amended complaint against Labeed Diab, Joe Jordan, John Larsen, John Maldonado, Carmine Petrone, Christopher Krubert, Joanne Burns and James Parisi (collectively, the “Legacy ATI Defendants”), Drew McKnight, Joshua Pack, Aaron Hood, Carmen Policy, Marc Furstein, Leslee Cowen, Rafeket Russak-Aminoach, and Sunil Gulati (collectively, the “FVACII Individual Defendants”), and Fortress Acquisition Sponsor II, LLC and Fortress Investment Group LLC (together, the "Fortress Entity Defendants," and together with the FVACII Individual Defendants, the “FVACII Defendants”). The consolidated amended complaint asserts claims on behalf of ATI against: (i) the FVACII Defendants for breach of fiduciary duty; (ii) Fortress Acquisition Sponsor II, LLC and the Legacy ATI Defendants for aiding and abetting breach of fiduciary duty; (iii) Labeed Diab, Joe Jordan, and Drew McKnight for contribution under Section 21D of the Exchange Act; (iv) the FVACII Defendants under Section 14(a) of the Exchange Act; (v) the Legacy ATI Defendants for unjust enrichment; and (vi) all defendants for contribution and indemnification under Delaware law. Plaintiffs' allegations generally mirror those asserted in the stockholder class action described above. On January 20, 2023, defendants filed motions to dismiss the consolidated amended complaint, which remain pending. On March 3, 2023, in lieu of filing a response to defendants' motions to dismiss, Plaintiffs filed a motion for leave to file an amended complaint, which was fully briefed as of April 7, 2023 and remains pending. The Company has determined that potential liabilities related to the action are not considered probable or reasonably estimable at this time. Insurance coverage complaint On March 8, 2023, the Company filed a complaint against Federal Insurance Company, U.S. Specialty Insurance Company and other insurers titled ATI Physical Therapy, Inc. v. Federal Insurance Company et. al., Case No. N23C-03-074, in the Superior Court of the State of Delaware related to a coverage dispute and those certain insurers’ denial of coverage for the stockholder class action complaints, the stockholder derivative complaint, and the SEC requests discussed in this section. The complaint asserts claims against Federal Insurance Company for breach of contract and bad faith, and claims for declaratory judgment as to Federal Insurance Company, U.S. Specialty Insurance Company, XL Specialty Insurance Company and the Company’s excess insurance carriers, seeking coverage for the stockholder class action complaints, the stockholder derivative complaint, and the SEC requests. On June 26, 2023, the Company filed an amended complaint asserting the same claims and seeking the same relief. On July 18, 2023, the defendants filed their answers to the amended complaint. On July 14, 2023, Federal Insurance Company issued a supplemental coverage position in which, subject to certain reservations and limitations, Federal Insurance Company accepted coverage for certain insureds with respect to the stockholder class action complaints and the stockholder derivative complaints. The insurance coverage litigation remains pending. During the third quarter of 2023, the Company began receiving insurance reimbursements for legal costs incurred related to the stockholder class action complaint and stockholder derivative complaint previously disclosed. The Company recognized $7.9 million of legal cost insurance reimbursements which is included as an offset to selling, general and administrative expenses in its consolidated statements of operations for the year ended December 31, 2023, of which $4.7 million was received in cash. Regulatory matters On November 5, 2021, the Company received from the SEC a voluntary request for the production of documents relating to the earnings forecast and financial information referenced in the Company's July 26, 2021 Form 8-K and related matters. The Company has subsequently received from the SEC additional requests for documents and information related to the same matters, and is cooperating with the SEC's review and investigation of those matters. Indemnifications The Company has agreed to indemnify its current and former directors and executive officers for costs associated with any fees, expenses, judgments, fines and settlement amounts incurred by them in any action or proceeding to which any of them are, or are threatened to be, made a party by reason of their service as a director or officer. The Company maintains director and officer insurance coverage that would generally enable it to recover a portion of any amounts paid. The ultimate cost of current or potential future litigation may exceed the Company’s current insurance coverages and may have a material adverse impact on our results of operations, cash flows and financial condition. The Company also may be subject to indemnification obligations by law with respect to the actions of its employees under certain circumstances and in certain jurisdictions.

Loss per Share

Loss per Share	12 Months Ended
Loss per Share	Dec. 31, 2023
Earnings Per Share [Abstract]
Loss per Share	Loss per Share Basic loss per share is computed by dividing loss available to common stockholders by the weighted average number of common shares outstanding during the period. Diluted loss per share is computed by dividing loss available to common stockholders by the weighted average number of common shares outstanding during the period, adjusted for the impact of securities that would have a dilutive effect on basic loss per share, if any. For the years ended December 31, 2023 and 2022, shares of Series A Senior Preferred Stock are treated as participating securities and therefore are included in computing earnings per common share using the two-class method. The two-class method is an earnings allocation formula that calculates basic and diluted net earnings per common share for each class of common stock separately based on dividends declared and participation rights in undistributed earnings as if the earnings for the year had been distributed. For the years ended December 31, 2023 and 2022, the loss available to common stockholders is increased by the amount of the cumulative dividend and any redemption value adjustments for the Series A Senior Preferred Stock that was issued as part of the 2022 Debt Refinancing. As discussed in Note 8 - Borrowings , the Series B Preferred Stock are non-economic and represent voting rights only and, therefore, are not considered in the calculation of basic or diluted loss per share. The calculation of both basic and diluted loss per share for the periods indicated below was as follows (in thousands, except per share data): Year Ended December 31, 2023 December 31, 2022 Basic and diluted loss per share: Net loss $ (66,078) $ (493,047) Less: Net income (loss) attributable to non-controlling interests 3,717 (668) Less: Series A Senior Preferred redemption value adjustments (1) 38,958 — Less: Series A Senior Preferred cumulative dividend 23,219 17,876 Loss available to common stockholders $ (131,972) $ (510,255) Weighted average shares outstanding (2) 4,133 4,063 Basic and diluted loss per share $ (31.93) $ (125.59) (1) For the year ended December 31, 2023, the Series A Senior Preferred Stock was remeasured to its redemption value. For the year ended December 31, 2023, this adjustment included a one-time recognition of a deemed dividend primarily from the original issue discount and an incremental redemption value adjustment to reflect the carrying amount equal to what the redemption amount would be as if redemption were to occur at the end of the reporting period. Refer to Note 11 - Mezzanine and Stockholders' Equity for additional information. (2) Included within weighted average shares outstanding following the 2022 Debt Refinancing are common shares issuable upon the exercise of the Series II Warrants, as the Series II Warrants are exercisable at any time for nominal consideration. As such, the shares are considered to be outstanding for the purpose of calculating basic and diluted loss per share. For the periods presented, basic and diluted loss per share were equal. The following number of shares issuable related to outstanding securities could potentially dilute earnings per share in the future (in thousands): Year Ended December 31, 2023 December 31, 2022 2L Notes (1) 8,625 — Series I Warrants 105 105 IPO Warrants 197 197 Restricted shares (2) 5 8 Stock options 99 106 RSUs 767 84 RSAs 1 4 Total 9,799 504 (1) Potential dilution is reflected on an if-converted basis based on the principal amount of 2L Notes as of the end of the periods presented, and Conversion Price of $12.50 per share. (2) Represents certain shares of Class A common stock legally issued, but not outstanding, as of the respective periods. As the vesting thresholds have not yet been met as of the end of th e reporting period, 0.3 million Earnout Shares and approximately 0.2 million Vesting Shares were excluded from the basic and diluted shares outstanding calculations.

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	12 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Pay vs Performance Disclosure
Net loss attributable to ATI Physical Therapy, Inc.	$ (69,795)	$ (492,379)

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
Insider Trading Arrangements	Dec. 31, 2023
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	The accompanying consolidated financial statements of the Company were prepared in accordance with U.S. generally accepted accounting principles ("GAAP") and in accordance with the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”). The Company meets the SEC's definition of a "Smaller Reporting Company," and therefore qualifies for the SEC's reduced disclosure requirements for smaller reporting companies.
Liquidity and going concern	Liquidity and going concern The accompanying consolidated financial statements have been prepared on a going concern basis, which contemplates the realization of assets and the satisfaction of liabilities in the normal course of business within twelve months after the date that these consolidated financial statements are issued. In the Company's consolidated financial statements as of and for the periods ended December 31, 2023 and 2022, these conditions and events continue to raise substantial doubt about the Company's ability to continue as a going concern. In response to these conditions, management plans included refinancing the Company's debt under its 2022 Credit Agreement (as defined in Note 8) and improving operating results and cash flows. Additionally, the Company experienced improvements in operations that resulted in reduced levels of operating cash outflows during the year ended December 31, 2023 relative to the same period in the prior year. A continued improvement in business results is necessary as there remains a risk that the Company may fail to meet its minimum liquidity covenant or be unable to fund anticipated cash requirements and obligations as they become due in the future. The Company's plan is to continue its efforts to improve its operating results and cash flow through increases to clinical staffing levels, improvements in clinician productivity, controlling costs and capital expenditures and increases in patient visit volumes, referrals and rate per visit. There can be no assurance that the Company's plan will be successful in any of these respects. If the Company's plan does not result in improvement in these aspects in future periods that results in sufficient cash flow from operations, the Company will need to consider other alternatives, such as raising additional financing, obtaining funds from other sources, disposal of assets, or pursuing other strategic alternatives to improve its business, results of operations and financial condition. There can be no assurance that the Company will be successful in accessing such alternative options or financing if or when needed. Failure to do so could have a material adverse impact on our business, financial condition, results of operations and cash flows, and may lead to events including bankruptcy, reorganization or insolvency. Management plans have not been fully implemented and, as a result, the Company has concluded that management's plans do not alleviate substantial doubt about the Company's ability to continue as a going concern. The consolidated financial statements do not include any adjustments relating to the recoverability and classification of recorded asset amounts or the amounts and classification of liabilities that might result from the outcome of this uncertainty.
Principles of consolidation	Principles of consolidation The consolidated financial statements include the financial statements of the Company, its subsidiaries, and entities for which the Company has a controlling financial interest, including variable interest entities ("VIEs") for which the Company is the primary beneficiary. All intercompany balances and transactions have been eliminated in consolidation, and net earnings are reduced by the portion of net earnings attributable to non-controlling interests.
Variable interest entities	Variable interest entities The Company consolidates all variable interest entities where the Company is the primary beneficiary. The Company identifies the primary beneficiary of a VIE as the enterprise that has both: (i) the power to direct the activities of the VIE that most significantly impact the entity's economic performance; and (ii) the obligation to absorb losses or the right to receive benefits of the VIE that could be significant to the entity. The Company may change the original assessment of a VIE upon subsequent events such as the modification of contractual agreements. The Company has an investment in RSFH-ATI Physical Therapy, LLC ("RSFH") that qualifies as a VIE. Based on the provisions of the RSFH agreement, the Company manages the entity and handles all day-to-day operating decisions in exchange for management fees and may receive distributions proportionate with its level of ownership. Accordingly, the Company has the decision-making power over the activities that most significantly impact the entity's economic performance and the obligation to absorb losses or the right to receive benefits that could be significant to the entity.
Use of estimates	Use of estimates The preparation of the consolidated financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting period. Actual results could differ from those estimates. The effect of any change in estimates will be recognized in the current period of the change.
Segment reporting	Segment reporting The Company reports segment information based on the management approach. The management approach designates the internal reporting used by management for making decisions and assessing performance as the source of the Company’s reportable segments. All of the Company’s operations are conducted within the United States. Our chief operating decision maker is our Chief Executive Officer, who reviews financial information presented on a consolidated basis for purposes of making decisions, assessing financial performance and allocating resources. We operate our business as one operating segment and therefore we have one reportable segment.
Cash, cash equivalents and restricted cash	Cash, cash equivalents and restricted cash
Accounts receivable	Accounts receivable The Company's accounts receivable are reported net of contractual allowances, denials allowances and allowances for doubtful accounts. The majority of accounts receivable are due from commercial insurance companies, workers' compensation plans, auto personal injury claims and government health programs, such as Medicare or Medicaid. The Company reports accounts receivable at an amount equal to the consideration the Company expects to receive in exchange for providing healthcare services to its patients.
Allowance and Provision for doubtful accounts	Allowance for doubtful accounts The Company estimates the allowance for doubtful accounts based upon several factors, including the age of the outstanding receivables, the historical experience of collections, the impact of economic conditions and, in some cases, evaluating specific customer accounts for the ability to pay. Management judgment is used to assess the collectability of accounts and the ability of the Company’s customers to pay. Provision for doubtful accounts
Concentrations of business risk	Concentrations of business risk
Net patient and other revenue	Net patient revenue Net patient revenue consists of revenue for physical therapy services. Net patient revenue is recognized at an amount equal to the consideration the Company expects to receive from third-party payors, patients and others for services rendered when the performance obligations under the terms of the contract are satisfied. There is a contract between the Company and the patient upon each visit resulting in the Company’s performance obligation to provide services to the patient. Generally, the performance obligation is satisfied as the patient receives physical therapy services provided by the Company, as each service provided is distinct and future services rendered are not dependent on previously rendered services. The Company has separate contractual agreements (written or implied) with third-party payors (e.g., insurers, managed care programs, government programs, workers' compensation) that provide for payments to the Company at amounts different from its established rates; the difference between the Company's established rates and amounts paid by third-party payors represent explicit price concessions in the form of contractual allowances or denials allowances. While these agreements are not considered contracts with the customer, they are used for determining the transaction price for services provided to the patients covered by the third-party payors. The payor contracts do not indicate performance obligations of the Company but indicate reimbursement rates for patients who are covered by those payors when the services are provided. To determine the transaction price associated with the contract, the Company includes the estimated effects of any variable consideration, such as contractual allowances, denials allowances and implicit price concessions. When the Company has written payment contracts with third-party payors with negotiated prices for services (contracted payors), the Company determines the transaction price using the negotiated contractual rates to estimate contractual allowances as compared to established rates; additional variable consideration for denial allowances is estimated using a portfolio approach that incorporates whether or not the Company has historical differences from negotiated contractual rates due to non-compliance with contract provisions. When the Company does not have written payment contracts with third-party payors (non-contracted payors), the Company determines the transaction price using a portfolio approach to estimate variable consideration for contractual allowances, denial allowances and implicit price concessions based on historical collections experience for claims with similar characteristics, such as location of service and type of third-party payor, in relation to its established rates. For both contracted and non-contracted payors, any subsequent changes in estimate of the transaction price is recorded as a revenue adjustment. Management believes that calculating at the portfolio level would not differ materially from considering each patient account separately. The Company continually reviews the transaction price estimation process to consider updates to laws and regulations and changes in third-party payor contractual terms that result from contract renegotiations and renewals. Due to complexities involved in determining amounts ultimately due under reimbursement arrangements with third-party payors and government entities, which are often subject to interpretation, the Company may receive reimbursement for healthcare services that is different from the estimates, and such differences could be material. Other revenue Revenue from the ATI Worksite Solutions ("AWS") business is derived from on-site services provided to clients’ employees including injury prevention, rehabilitation, ergonomic assessments and performance optimization. The Company accounts for AWS services as single performance obligations satisfied over time. Revenue from AWS is recognized when obligations under the terms of the contract are satisfied. Revenues are recognized at an amount equal to the consideration the Company expects to receive in exchange for providing services to its clients, which is variable in nature and determined based on the number of hours and respective rate for services provided in a given period. Revenues from Management Service Agreements (“MSA”) are derived from contractual arrangements whereby the Company manages a non-controlled clinic or clinics for third-party owners. The Company does not have any ownership interest in these clinics. The Company accounts for MSA services as single performance obligations satisfied over time. Typically, the transaction price is variable in nature and revenue is determined based on the number of visits conducted at the clinic and recognized when services per the contractual arrangements are performed. Costs, primarily salaries for the Company’s employees, are recorded when incurred. Other revenue includes physical or occupational therapy services and athletic training provided on-site, such as at schools and industrial worksites. Contract terms and rates are agreed to in advance between the Company and the third-parties. Services are typically performed over the contract period, and revenue is recorded as services are performed. If the services are paid in advance, revenue is deferred and recognized as the services are performed.
Property and equipment	Property and equipment Property and equipment acquired is recorded at cost less accumulated depreciation, except during an acquisition of a business, in which case the assets are initially recorded at fair value. Depreciation is calculated using the straight-line method and is provided in amounts sufficient to attribute the cost of depreciable assets to operations over the estimated useful lives. The approximate useful life of each class of property and equipment is as follows: Equipment 3 - 5 years Furniture & fixtures 5 - 7 years Automobiles 3 - 5 years Software 3 - 5 years Buildings 40 years Leasehold improvements Lesser of lease term or estimated useful lives of the assets (generally 5 - 15 years) Major repairs that extend the useful life of an asset are capitalized to the property and equipment account. Routine maintenance and repairs are charged to rent, clinic supplies, contract labor and other expenses and selling, general and administrative expenses. Gains or losses associated with property and equipment retired or sold are included in earnings. Computer software is included in property and equipment and consists of purchased software and internally developed software. The Company capitalizes application-stage development costs for significant internally developed software projects. Once the software is ready for its intended use, these costs are amortized on a straight-line basis over the software’s estimated useful life. Costs recognized in the preliminary project phase and the post-implementation phase, as well as maintenance and training costs, are expensed as incurred.
Impairment of long-lived assets	Impairment of long-lived assets
Goodwill and intangible assets	Goodwill and intangible assets Goodwill represents the excess of the purchase price over the fair value of assets acquired and liabilities assumed. The Company accounts for goodwill and indefinite-lived intangible assets under Accounting Standards Codification (“ASC”) Topic 350, Intangibles – Goodwill and Other , which requires the Company to test goodwill and other indefinite-lived assets for impairment annually or whenever events or circumstances indicate that impairment may exist. The Company did not note any triggering events during 2023 that resulted in the recording of an impairment loss, but noted triggering events during 2022 which resulted in the recording of impairment losses. Refer to Note 5 - Goodwill, Trade Name and Other Intangible Assets for further details. Due to the current economic uncertainty resulting from rising interest rates, inflation and other macroeconomic factors, the Company will continue to review the carrying amounts of goodwill and indefinite-lived assets for potential triggering events. The cost of acquired businesses is allocated first to its identifiable assets, both tangible and intangible, based on estimated fair values. Costs allocated to finite-lived identifiable intangible assets are generally amortized on a straight-line basis over the remaining estimated useful lives of the assets. The excess of the purchase price over the fair value of identifiable assets acquired, net of liabilities assumed, is recorded as goodwill. The approximate useful life of each class of intangible asset is as follows: ATI Physical Therapy trade name/trademark Indefinite Non-compete agreements 2 - 5 years Other intangible assets 15 years Goodwill and intangible assets with indefinite lives are not amortized but must be reviewed at least annually for impairment. If the impairment test indicates that the carrying value of the reporting unit exceeds its fair value, then a goodwill impairment loss should be recognized in the consolidated statements of operations in an amount equal to the excess carrying value over fair value. If the impairment test indicates that the carrying value of an intangible asset exceeds its fair value, then an impairment loss should be recognized in the consolidated statements of operations in an amount equal to the excess carrying value over fair value. Fair value is determined using valuation techniques based on estimates, judgments and assumptions the Company believes are appropriate in the circumstances. The Company completed the annual impairment analysis of goodwill as of October 1, 2023 by estimating its fair value using an average of a discounted cash flow analysis and comparable public company analysis. The key assumptions associated with determining the estimated fair value include projected revenue growth rates and EBITDA margins, the terminal growth rate, the discount rate and relevant market multiples. The Company completed the annual impairment analysis of the indefinite-lived intangible asset as of October 1, 2023 by estimating its fair value using the relief from royalty method. The key assumptions associated with determining the estimated fair value include projected revenue growth rates, the royalty rate, the discount rate and the terminal growth rate.
Deferred financing costs and 2L Notes	Deferred financing costs 2L Notes The guidance in ASC Topic 825, Financial Instruments , provides a fair value option that allows companies to make an irrevocable election of fair value as the initial and subsequent measurement attribute for certain eligible financial assets and liabilities. Unrealized gains and losses on items for which the fair value option has been elected are reported in earnings. The decision to elect the fair value option is determined on an instrument-by-instrument basis, must be applied to an entire instrument and is irrevocable once elected. Assets and liabilities measured at fair value pursuant to this guidance are required to be reported separately in the Company's consolidated balance sheets from those instruments using another accounting method. The 2L Notes are accounted for as a liability in the Company's consolidated balance sheets. The Company has made an irrevocable election to account for the 2L Notes under the fair value option in accordance with ASC Topic 825, Financial Instruments
Preferred stock	Preferred stock Preferred stock is classified as debt, equity or mezzanine equity based on its redemption features. Preferred stock with redemption features outside of the control of the issuer, such as contingent redemption features, is classified as mezzanine equity. Preferred stock with mandatory redemption features is classified as debt. Preferred stock with no redemption features, or redemption features over which the issuer has control, is classified as equity. The Company has preferred stock that is classified as mezzanine equity in the Company's consolidated balance sheets. Refer to Note 11 - Mezzanine and Stockholders' Equity for more information about the Company’s outstanding Series A Senior Preferred Stock.
Treasury stock	Treasury stock Treasury stock amounts are accounted for under the cost method whereby the entire cost of the acquired stock is recorded as treasury stock. Gains and losses on the subsequent reissuance of shares are credited or charged to paid-in capital in excess of par value using the average-cost method.
Warrant liability	Warrant liability The Company accounts for its outstanding IPO Warrants in accordance with the guidance contained in ASC Topic 815-40, Derivatives and Hedging - Contracts on an Entity’s Own Equity , and determined that the IPO Warrants do not meet the criteria for equity treatment thereunder. As such, each IPO Warrant must be recorded as a liability and is subject to re-measurement at each balance sheet date. Changes in fair value are recognized in change in fair value of warrant liability in the Company’s consolidated statements of operations.
Contingent common shares liability	Contingent common shares liability The Company accounts for its potential Earnout Shares and Vesting Shares as a liability in accordance with the guidance in ASC Topic 480, Distinguishing Liabilities from Equity , and ASC Topic 815, Derivatives and Hedging, and is subject to re-measurement at each balance sheet date. Changes in fair value are recognized in change in fair value of contingent common shares liability in the Company’s consolidated statements of operations.
Non-controlling interests in consolidated affiliates	Non-controlling interests in consolidated affiliates
Fair value of financial instruments	Fair value of financial instruments The Company determines fair value measurements used in its consolidated financial statements based upon the exit price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants exclusive of any transaction costs, as determined by either the principal market or the most advantageous market at the measurement date. Inputs used in the valuation techniques to derive fair values are classified based on a three-level hierarchy. The basis for fair value measurements for each level within the hierarchy is described below with Level 1 having the highest priority and Level 3 having the lowest. • Level 1: Observable inputs, which include unadjusted quoted prices in active markets for identical instruments. • Level 2: Observable inputs other than Level 1 inputs, such as quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the instruments. • Level 3: Unobservable inputs in which little or no market data exists, therefore requiring an entity to develop its own assumptions, such as valuations derived from valuation techniques in which one or more significant inputs or significant value drivers are unobservable. Refer to Note 14 - Fair Value Measurements for valuation techniques and inputs related to the Company's financial instruments and share-based liabilities.
Income taxes	Income taxes The Company accounts for income taxes in accordance with ASC Topic 740 (“ASC 740”), Income Taxes . Under ASC 740, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and the respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rate is recognized in operations in the period that includes the enactment date. ASC 740 provides guidance on how uncertain tax positions should be recognized, measured, presented and disclosed in the financial statements. ASC 740 requires the evaluation of tax positions taken or expected to be taken in the course of preparing the Company’s tax returns to determine whether the tax positions are more likely than not of being sustained by the applicable tax authority. A tax position that meets the more-likely-than-not recognition threshold is measured and recognized in the consolidated financial statements at the largest amount of benefit that is greater than 50% likely of being realized upon settlement. Tax positions deemed to not meet a more-likely-than-not threshold may not be recognized in the financial statements. The Company reviews these tax uncertainties in light of changing facts and circumstances, such as the progress of tax audits, and if any tax uncertainties were identified, the Company would recognize them accordingly. The liability relating to uncertain tax positions is classified as current in the consolidated balance sheets to the extent the Company anticipates making a payment within one year. The Company did not identify any uncertain tax positions in the years ended December 31, 2023 and December 31, 2022. Interest and penalties associated with income taxes are classified in the income tax expense (benefit) line in the consolidated statements of operations.
Cost of services	Cost of services Cost of services consist of salaries specific to the Company’s clinic operations along with rent, clinic supplies expense, depreciation and advertising costs. In addition, cost of services includes the provision for doubtful accounts.
Selling, general and administrative expenses	Selling, general and administrative expenses Selling, general and administrative expenses consist primarily of wages and benefits for corporate personnel, corporate outside services, marketing costs, depreciation of corporate fixed assets, amortization of intangible assets and certain corporate level professional fees, including those related to legal, accounting and payroll.
Advertising costs	Advertising costs
Share-based compensation	Share-based compensation The Company applies the guidance in ASC Topic 718, Compensation - Stock Compensation , in its accounting for share-based compensation. The Company recognizes compensation expense for all share-based compensation awarded to employees, net of forfeitures, using a fair value-based method. The grant-date fair value of each award is amortized to expense on a straight-line basis over the award’s vesting period. Compensation expense associated with share-based awards is included in salaries and related costs and selling, general and administrative expenses in the accompanying consolidated statements of operations, depending on whether the award recipient is a clinic-level or corporate employee, respectively. Share-based compensation expense is adjusted for forfeitures as incurred.
Loss per share	Loss per share The Company applies the guidance in ASC Topic 260, Earnings Per Share , in its computation of loss per share. Basic loss per share is computed by dividing loss available to common stockholders by the weighted average number of common shares outstanding during the period. Diluted loss per share is computed by dividing loss available to common stockholders by the weighted average number of common shares outstanding during the period, adjusted for the impact of securities that would have a dilutive effect on basic loss per share, if any. Refer to Note 18 - Loss per Share for more information.
Leases	Leases The Company applies the guidance in ASC Topic 842 (“ASC 842”), Leases, to classify individual leases of assets as either operating or finance leases at contract inception. All leased assets have been classified as operating lease arrangements, and the Company’s classes of leased assets include real estate and equipment. Operating lease balances are included in operating lease right-of-use (“ROU”) assets, current portion of operating lease liabilities and operating lease liabilities in the Company’s consolidated balance sheets. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent its obligation to make lease payments arising from the lease. ROU assets and lease liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term, adjusted for prepaid or accrued lease payments and lease incentives. The Company’s lease terms include the impact of options to extend or terminate the lease when it is reasonably certain that the options will be exercised or not exercised, as appropriate. When discount rates implicit in leases cannot be readily determined, the Company uses the applicable incremental borrowing rate at lease commencement to perform lease classification tests on lease components and to measure lease liabilities and ROU assets. The Company's incremental borrowing rate is the rate of interest that it would have to pay to borrow on a collateralized basis, over a similar term, an amount equal to the lease payments in a similar economic environment. Leases with an initial term of 12 months or less are not recorded on the balance sheet. The ROU asset is subject to testing for impairment if there is an indicator for impairment, in accordance with the impairment accounting guidance for long-lived assets. The Company noted triggering events during 2023 and 2022 which resulted in the recording of ROU asset impairment losses of $2.3 million and $2.6 million for the years ended December 31, 2023 and 2022, respectively, which are included in total long-lived asset impairment losses. The amortization of operating lease ROU assets and the accretion of operating lease liabilities are reported together as fixed lease expense. The fixed lease expense is recognized on a straight-line basis over the life of the lease. If the ROU asset has been impaired, lease expense is no longer recognized on a straight-line basis. The lease liability continues to amortize using the effective interest method, while the ROU asset is subsequently amortized on a straight-line basis. Some of the Company’s operating leases include variable lease payments, which include periodic adjustments of the Company's payments for the use of the asset based on changes in factors such as consumer price indices, fair market value, tax rates imposed by taxing authorities or lessor cost of insurance. To the extent they are not included in operating lease liabilities and operating lease ROU assets, these variable lease payments are recognized as incurred. Additionally, the Company makes payments for property taxes, insurance, common area maintenance or other services and accounts for these costs as variable lease payments since the Company elected the practical expedient within ASC 842 to not separate lease and non-lease components within lease transactions for all classes of assets.
Recently adopted accounting guidance	Recently adopted accounting guidance In October 2021, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2021-08, Business Combinations (Topic 805): Accounting for Contract Assets and Liabilities from Contracts with Customers , which provides guidance to improve the accounting for acquired revenue contracts with customers in a business combination by addressing diversity in practice. This ASU is effective for the Company on January 1, 2023, with early adoption permitted, and shall be applied on a prospective basis to business combinations that occur on or after the adoption date. The Company adopted this new accounting standard effective January 1, 2023. The adoption of this standard did not have a material impact on the Company's consolidated financial statements. Recent accounting pronouncements In November 2023, the FASB issued ASU 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures , which provides guidance to improve the disclosures for reportable segments through enhanced disclosures about significant segment expenses. This ASU is effective for the Company's annual financial statements to be issued for the year ended December 31, 2024, and the Company's interim financial statements during the year ended December 31, 2025, with early adoption permitted. This ASU shall be applied on a retrospective basis for all prior periods presented in the financial statements. The Company expects to adopt this new accounting standard in its Annual Report on Form 10-K for the year ended December 31, 2024, and does not expect the adoption of this standard to have a material impact on the Company's consolidated financial statements. In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures , which provides guidance to improve the disclosures for income taxes primarily through enhanced rate reconciliation and income taxes paid disclosures. This ASU is effective for the Company's annual financial statements to be issued for the year ended December 31, 2025, with early adoption permitted, and shall be applied on a prospective basis. The Company expects to adopt this new accounting standard in its Annual Report on Form 10-K for the year ended December 31, 2025, and does not expect the adoption of this standard to have a material impact on the Company's consolidated financial statements.

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Property and Equipment	The approximate useful life of each class of property and equipment is as follows: Equipment 3 - 5 years Furniture & fixtures 5 - 7 years Automobiles 3 - 5 years Software 3 - 5 years Buildings 40 years Leasehold improvements Lesser of lease term or estimated useful lives of the assets (generally 5 - 15 years) Property and equipment consisted of the following at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Equipment $ 37,980 $ 38,102 Furniture and fixtures 14,311 17,215 Leasehold improvements 178,888 191,182 Automobiles 4 19 Computer equipment and software 108,749 102,651 Construction-in-progress 2,134 3,727 342,066 352,896 Accumulated depreciation and amortization (241,644) (229,206) Property and equipment, net (1) $ 100,422 $ 123,690 (1) Excludes $0.7 million and $1.1 million reclassified as held for sale as of December 31, 2023 and December 31, 2022, respectively. Refer to Note 3 - Divestitures for additional information. The following table presents the amount of depreciation and amortization expense related to property and equipment recorded in rent, clinic supplies, contract labor and other and selling, general and administrative expenses in the Company’s consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Rent, clinic supplies, contract labor and other $ 25,179 $ 27,429 Selling, general and administrative expenses 11,509 12,417 Total depreciation expense $ 36,688 $ 39,846
Schedule of Carrying Amounts of Indefinite-Lived Intangible Assets	The approximate useful life of each class of intangible asset is as follows: ATI Physical Therapy trade name/trademark Indefinite Non-compete agreements 2 - 5 years Other intangible assets 15 years The table below summarizes the Company’s carrying amount of trade name and other intangible assets at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Gross intangible assets: ATI trade name (1) $ 245,000 $ 245,000 Non-compete agreements 2,395 2,395 Other intangible assets 640 640 Accumulated amortization: Accumulated amortization – non-compete agreements (1,807) (1,126) Accumulated amortization – other intangible assets (370) (327) Total trade name and other intangible assets, net $ 245,858 $ 246,582 (1) Not subject to amortization.
Schedule of Carrying Amounts of Finite-Lived Intangible Assets	The approximate useful life of each class of intangible asset is as follows: ATI Physical Therapy trade name/trademark Indefinite Non-compete agreements 2 - 5 years Other intangible assets 15 years The table below summarizes the Company’s carrying amount of trade name and other intangible assets at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Gross intangible assets: ATI trade name (1) $ 245,000 $ 245,000 Non-compete agreements 2,395 2,395 Other intangible assets 640 640 Accumulated amortization: Accumulated amortization – non-compete agreements (1,807) (1,126) Accumulated amortization – other intangible assets (370) (327) Total trade name and other intangible assets, net $ 245,858 $ 246,582 (1) Not subject to amortization.

Divestitures (Tables)

Divestitures (Tables)	12 Months Ended
Divestitures (Tables)	Dec. 31, 2023
Discontinued Operations and Disposal Groups [Abstract]
Schedule of Disposal Groups, Including Discontinued Operations	Major classes of assets and liabilities classified as held for sale as of December 31, 2023 and December 31, 2022 were as follows (in thousands): December 31, 2023 December 31, 2022 Accounts receivable, net $ — $ 486 Prepaid expenses — 23 Property and equipment, net 674 1,113 Operating lease right-of-use assets 1,382 1,929 Goodwill, net — 3,192 Other non-current assets — 12 Total assets held for sale $ 2,056 $ 6,755 Accounts payable $ — $ 22 Accrued expenses and other liabilities — 201 Current portion of operating lease liabilities 357 685 Operating lease liabilities 1,421 1,706 Total liabilities held for sale $ 1,778 $ 2,614

Revenue from Contracts with C_2

Revenue from Contracts with Customers (Tables)	12 Months Ended
Revenue from Contracts with Customers (Tables)	Dec. 31, 2023
Revenue from Contract with Customer [Abstract]
Schedule of Disaggregation of Net Operating Revenue By Major Service Line and Associated Payor Class	The following table disaggregates net revenue by major service line for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Net patient revenue $ 636,095 $ 575,940 ATI Worksite Solutions (1) 37,219 35,515 Management Service Agreements (1) 14,831 12,857 Sports Medicine and other revenue (1) 10,871 11,359 $ 699,016 $ 635,671 (1) ATI Worksite Solutions, Management Service Agreements and Sports Medicine and other revenue are included within other revenue on the face of the consolidated statements of operations. The following table disaggregates net patient revenue for each associated payor class as a percentage of total net patient revenue for the periods indicated below: Year Ended December 31, 2023 December 31, 2022 Commercial 58.6 % 57.6 % Government 23.2 % 24.2 % Workers’ compensation 11.7 % 12.4 % Other (1) 6.5 % 5.8 % 100.0 % 100.0 % (1) Other is primarily comprised of net patient revenue related to auto personal injury reimbursement.

Goodwill, Trade Name and Othe_2

Goodwill, Trade Name and Other Intangible Assets (Tables)	12 Months Ended
Goodwill, Trade Name and Other Intangible Assets (Tables)	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Changes in The Carrying Amount of Goodwill	Changes in the carrying amount of goodwill during the current year consisted of the following (in thousands): Goodwill at December 31, 2022 (1) $ 286,458 Impairment charges (2) — Reclassifications to held and used 3,192 Goodwill at December 31, 2023 (1) $ 289,650 (1) Net of accumulated impairment losses of $1,045.7 million. (2) The Company did not note any triggering events during the year ended December 31, 2023 that resulted in the recording of an impairment loss.
Schedule of Carrying Amounts of Indefinite-Lived Intangible Assets	The approximate useful life of each class of intangible asset is as follows: ATI Physical Therapy trade name/trademark Indefinite Non-compete agreements 2 - 5 years Other intangible assets 15 years The table below summarizes the Company’s carrying amount of trade name and other intangible assets at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Gross intangible assets: ATI trade name (1) $ 245,000 $ 245,000 Non-compete agreements 2,395 2,395 Other intangible assets 640 640 Accumulated amortization: Accumulated amortization – non-compete agreements (1,807) (1,126) Accumulated amortization – other intangible assets (370) (327) Total trade name and other intangible assets, net $ 245,858 $ 246,582 (1) Not subject to amortization.
Schedule of Carrying Amounts of Finite-Lived Intangible Assets	The approximate useful life of each class of intangible asset is as follows: ATI Physical Therapy trade name/trademark Indefinite Non-compete agreements 2 - 5 years Other intangible assets 15 years The table below summarizes the Company’s carrying amount of trade name and other intangible assets at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Gross intangible assets: ATI trade name (1) $ 245,000 $ 245,000 Non-compete agreements 2,395 2,395 Other intangible assets 640 640 Accumulated amortization: Accumulated amortization – non-compete agreements (1,807) (1,126) Accumulated amortization – other intangible assets (370) (327) Total trade name and other intangible assets, net $ 245,858 $ 246,582 (1) Not subject to amortization.

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
Property and Equipment (Tables)	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment and Depreciation Expense	The approximate useful life of each class of property and equipment is as follows: Equipment 3 - 5 years Furniture & fixtures 5 - 7 years Automobiles 3 - 5 years Software 3 - 5 years Buildings 40 years Leasehold improvements Lesser of lease term or estimated useful lives of the assets (generally 5 - 15 years) Property and equipment consisted of the following at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Equipment $ 37,980 $ 38,102 Furniture and fixtures 14,311 17,215 Leasehold improvements 178,888 191,182 Automobiles 4 19 Computer equipment and software 108,749 102,651 Construction-in-progress 2,134 3,727 342,066 352,896 Accumulated depreciation and amortization (241,644) (229,206) Property and equipment, net (1) $ 100,422 $ 123,690 (1) Excludes $0.7 million and $1.1 million reclassified as held for sale as of December 31, 2023 and December 31, 2022, respectively. Refer to Note 3 - Divestitures for additional information. The following table presents the amount of depreciation and amortization expense related to property and equipment recorded in rent, clinic supplies, contract labor and other and selling, general and administrative expenses in the Company’s consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Rent, clinic supplies, contract labor and other $ 25,179 $ 27,429 Selling, general and administrative expenses 11,509 12,417 Total depreciation expense $ 36,688 $ 39,846

Accrued Expenses and Other Li_2

Accrued Expenses and Other Liabilities (Tables)	12 Months Ended
Accrued Expenses and Other Liabilities (Tables)	Dec. 31, 2023
Payables and Accruals [Abstract]
Summary of Accrued Expenses and Other Liabilities	Accrued expenses and other liabilities consisted of the following at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Salaries and related costs $ 37,630 $ 28,949 Accrued legal settlement (1) 21,324 1,015 Credit balances due to patients and payors 7,712 6,117 Accrued professional fees 4,146 4,536 Accrued interest 4,913 762 Accrued occupancy costs 2,593 2,410 Accrued contract labor 2,255 4,483 Other payables and accrued expenses 7,862 5,400 Total $ 88,435 $ 53,672 (1) Includes estimated liability of $20.0 million related to settlement agreement in principle as of December 31, 2023. Refer to Note 17 - Commitments and Contingencies for additional information.

Borrowings (Tables)

Borrowings (Tables)	12 Months Ended
Borrowings (Tables)	Dec. 31, 2023
Debt Disclosure [Abstract]
Summary of Long-Term Debt	Long-term debt, net consisted of the following at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Senior Secured Term Loan (1, 2) (due February 24, 2028) $ 410,048 $ 503,481 Revolving Loans (3) (due February 24, 2027) 38,450 48,200 Less: unamortized debt issuance costs (7,395) (11,137) Less: unamortized original issue discount (7,525) (8,944) Total debt, net 433,578 531,600 Less: current portion of long-term debt — — Long-term debt, net $ 433,578 $ 531,600 (1) Interest rate of 12.7% and 12.1% at December 31, 2023 and December 31, 2022, respectively, with interest payable in designated installments at a variable interest rate. The effective interest rate for the Senior Secured Term Loan was 13.9% and 13.1% at December 31, 2023 and December 31, 2022, respectively. (2) The Company has paid a portion of its interest in-kind on its Senior Secured Term Loan by capitalizing and adding such interest to the principal amount of the debt. As of December 31, 2023 and December 31, 2022, the Company has recognized total paid-in-kind interest in the amount of $10.0 million and $3.5 million, respectively. (3) Weighted average interest rate of 9.5% and 8.3% at December 31, 2023 and December 31, 2022, respectively, with interest payable in designated installments at a variable interest rate. 2L Notes due to related parties, at fair value consisted of the following at December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 2L Notes due to related parties, at fair value $ 79,472 $ — The following table presents changes in the principal amount of the 2L Notes since the Closing Date (in thousands): December 31, 2023 2L Notes, principal amount at Closing Date $ 103,243 Paid-in-kind interest added during period 4,569 2L Notes, principal amount at end of period $ 107,812
Schedule of Debt Conversions For Voting Rights	The following table presents approximate changes in outstanding shares of Series B Preferred Stock since the Closing Date and associated equivalent common stock voting rights at the end of the period (in thousands): December 31, 2023 Series B Preferred Stock, shares at Closing Date 103 Increase (decrease) in shares during period 5 Series B Preferred Stock, shares at end of period 108 Common stock voting rights, as converted basis (1) 8,377 (1) Represents approximate shares of Series B Preferred Stock outstanding at end of period, times $1,000, divided by the contractual Voting Rights Conversion Price of $12.87 per share.
Schedule of Aggregate Maturities of Long-Term Debt	Aggregate maturities of the Company's borrowings at December 31, 2023 are as follows (in thousands): 2024 $ — 2025 — 2026 — 2027 38,450 2028 517,860 Thereafter — Total future maturities (1) 556,310 Unamortized original issue discount and debt issuance costs (14,920) 2L Notes due to related parties, principal amount (1, 2) (107,812) Long-term debt, net (1) $ 433,578 (1) Excludes any contractual paid-in-kind interest that may be accrued and added to the principal amounts between now and the respective maturity dates. (2) The principal amount of the 2L Notes differs from the estimated fair value presented on the consolidated balance sheet. Refer to Note 14 - Fair Value Measurements for further details on the fair value of the 2L Notes.

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	12 Months Ended
Employee Benefit Plans (Tables)	Dec. 31, 2023
Retirement Benefits [Abstract]
Schedule of Defined Contribution Plan Disclosures	The following table presents the Company’s matching contributions to the plan recorded in salaries and related costs and selling, general and administrative expenses in the consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Salaries and related costs $ 4,803 $ 4,374 Selling, general and administrative expenses 592 559 Total $ 5,395 $ 4,933

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
Share-Based Compensation (Tables)	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Schedule of Share-Based Payment Arrangement, Option, Activity	The following table summarizes the activity of stock options for the year ended December 31, 2023 (aggregate intrinsic value in thousands): Number of Options Weighted-Average Exercise Price Weighted-Average Contractual Term (in years) Aggregate Intrinsic Value Outstanding, January 1, 2023 106,495 $ 92.14 9.1 $ — Granted — — N/A N/A Adjustments 2,676 87.00 N/A N/A Exercised — — N/A — Forfeited/Cancelled (9,886) 96.46 N/A N/A Outstanding, December 31, 2023 99,285 $ 91.73 8.2 $ — Exercisable, December 31, 2023 32,181 $ 99.93 8.1 $ — Unvested, December 31, 2023 67,104 $ 87.80 8.2 $ —
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions	The following table summarizes the weighted-average grant-date fair value and assumptions used to develop the fair value estimates for the options granted in 2022. No stock options were granted under the 2021 Plan during the year ended December 31, 2023: 2022 Weighted-average grant-date fair value of options $48.79 Risk-free interest rate 1.74% Term (years) 6.2 Volatility 61.19% Expected dividend —%
Schedule of Share-Based Payment Arrangement, Restricted Stock Unit, Activity	The following table summarizes the activity of unvested RSUs and the respective weighted-average grant date fair value per RSU for the year ended December 31, 2023: 2023 RSUs Weighted-Average Grant Date Fair Value Outstanding and unvested, beginning of year 84,283 $ 99.46 Granted 762,201 16.58 Vested (39,145) 101.33 Forfeited (39,975) 30.22 Outstanding and unvested, end of year 767,364 $ 20.56
Nonvested Restricted Stock Shares Activity	The following table summarizes the activity of unvested RSAs and respective weighted-average grant date fair value per RSA for the year ended December 31, 2023: 2023 RSAs Weighted-Average Grant Date Fair Value Outstanding and unvested, beginning of year 3,391 $ 171.00 Granted — — Vested (2,233) 171.00 Forfeited (83) 171.00 Outstanding and unvested, end of year 1,075 $ 171.00

Mezzanine and Stockholders' E_2

Mezzanine and Stockholders' Equity (Tables)	12 Months Ended
Mezzanine and Stockholders' Equity (Tables)	Dec. 31, 2023
Equity [Abstract]
Schedule of Temporary Equity	The following table reflects the components of the initial proceeds related to the Series A Senior Preferred Stock (in thousands): Gross proceeds allocated to Series A Senior Preferred Stock $ 144,667 Less: original issue discount (1,447) Less: issuance costs (2,880) Net proceeds received from issuance of Series A Senior Preferred Stock $ 140,340 Changes in the aggregate stated value and stated value per share of the Series A Senior Preferred Stock consisted of the following (in thousands, except per share data): December 31, 2023 December 31, 2022 Aggregate stated value, beginning of period $ 182,876 $ 165,000 Paid-in-kind dividends (1) 23,219 17,876 Aggregate stated value, end of period $ 206,095 $ 182,876 Preferred shares issued and outstanding, end of period 165 165 Stated value per share, end of period $ 1,249.06 $ 1,108.34 (1) Changes in the stated value for the year ended December 31, 2022 represent changes since the Refinancing Date, which is when the Series A Senior Preferred Stock was issued and established. Changes in the carrying value of the Series A Senior Preferred Stock consisted of the following for the year ended December 31, 2023 (in thousands). There were no changes in carrying value in 2022. December 31, 2023 Carrying value, beginning of period $ 140,340 Write off original issue discount 1,447 Write off issuance costs 2,880 Deemed dividend from discount on initial gross proceeds allocation 20,333 Paid-in-kind dividends recognized to carrying value 41,095 Redemption value adjustment 14,298 Carrying value, end of period $ 220,393
Schedule of Components Of Proceeds Related to Warrants	The following table reflects the components of proceeds related to the 2022 Warrants (in thousands): Series I Warrants Series II Warrants Total Gross proceeds allocated to 2022 Warrants $ 5,101 $ 15,232 $ 20,333 Less: original issue discount (51) (152) (203) Less: issuance costs (102) (303) (405) Net proceeds received from issuance of 2022 Warrants $ 4,948 $ 14,777 $ 19,725
Schedule of Shares of Class A Common Stock Reserved for Potential Future Issuance	As of December 31, 2023, shares of Class A common stock reserved for potential future issuance, on an as-if converted basis, were as follows (in thousands): December 31, 2023 2L Notes (1) 8,625 Shares available for grant under the 2021 Plan 241 2021 Plan share-based awards outstanding 868 Earnout Shares reserved 300 2022 Warrant shares reserved 230 IPO Warrant shares reserved 197 Vesting Shares reserved (2) 173 Restricted shares (2) 5 Total shares of common stock reserved 10,639 (1) Calculated based on the principal amount of 2L Notes and Conversion Price of $12.50 per share. This figure differs from the contractual Voting Rights Conversion Price of $12.87 as outlined in Note 8 - Borrowings. (2) Represents shares of Class A common stock legally issued, but not outstanding, as of December 31, 2023.

IPO Warrant Liability (Tables)

IPO Warrant Liability (Tables)	12 Months Ended
IPO Warrant Liability (Tables)	Dec. 31, 2023
Other Liabilities Disclosure [Abstract]
Summary of Warrant Liability	The following table presents the change in the fair value of Private Placement Warrants that is recognized in change in fair value of warrant liability in the consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Fair value, beginning of period $ 29 $ 1,305 Decrease in fair value (28) (1,276) Fair value, end of period $ 1 $ 29 The following table presents the changes in the fair value of the Public Warrants that is recognized in change in fair value of warrant liability in the consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Fair value, beginning of period $ 69 $ 3,036 Decrease in fair value (67) (2,967) Fair value, end of period $ 2 $ 69

Contingent Common Shares Liab_2

Contingent Common Shares Liability (Tables)	12 Months Ended
Contingent Common Shares Liability (Tables)	Dec. 31, 2023
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Change in Fair Value of Earn Out Shares	The following table presents the changes in the fair value of the Earnout Shares that is recognized in change in fair value of contingent common shares liability in the consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Fair value, beginning of period $ 1,800 $ 28,800 Decrease in fair value (1,433) (27,000) Fair value, end of period $ 367 $ 1,800 The following table presents the changes in the fair value of the Vesting Shares that is recognized in change in fair value of contingent common shares liability in the consolidated statements of operations for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Fair value, beginning of period $ 1,035 $ 16,560 Decrease in fair value (824) (15,525) Fair value, end of period $ 211 $ 1,035

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
Fair Value Measurements (Tables)	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Schedule of Key Fair Value Measurement Inputs	The key inputs into the respective valuation models used to estimate the fair value of the 2L Notes were as follows as of December 31, 2023 and the Closing Date, which is when the 2L Notes were issued: 2L Notes December 31, 2023 June 15, 2023 Risk-free interest rate 3.83% 3.90% Volatility 45.00% 50.00% Selected yield 20.50% 20.00% Expected term (years) 4.7 5.3 Share price $6.14 $10.21 The key inputs into the Monte-Carlo option pricing model were as follows as of December 31, 2023 and December 31, 2022 for the respective Level 3 instruments: IPO Warrants Earnout Shares and Vesting Shares December 31, 2023 December 31, 2022 December 31, 2023 December 31, 2022 Risk-free interest rate 4.09% N/A 3.84% 3.88% Volatility 98.10% N/A 77.20% 74.60% Dividend yield —% N/A —% —% Expected term (years) 2.5 N/A 7.5 8.5 Share price $6.14 N/A $6.14 $15.50
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation	The following table presents the changes in the fair value of the 2L Notes that is recognized in change in fair value of 2L Note s in the consolidated statements of operations for the periods indicated below (in thousands). None of the change in fair value is attributable to instrument-specific credit risk: Year Ended December 31, 2023 Fair value, beginning of period (1) $ 103,943 Decrease in fair value (1) (24,471) Fair value, end of period $ 79,472 (1) Represents changes in fair value from the Closing Date, which is when the 2L Notes were issued.
Schedule of Cash Flow Hedges Included in Accumulated Other Comprehensive Income (Loss)	The following table presents the activity of cash flow hedges included in accumulated other comprehensive income (loss) for the years ended December 31, 2023 and 2022, respectively (in thousands): Cash Flow Hedges Balance as of December 31, 2022 $ 4,899 Unrealized gain recognized in other comprehensive income before reclassifications 801 Reclassification to interest expense, net (5,294) Balance as of December 31, 2023 $ 406 Balance as of December 31, 2021 $ 28 Unrealized gain recognized in other comprehensive income before reclassifications 8,310 Reclassification to interest expense, net (3,439) Balance as of December 31, 2022 $ 4,899
Schedule of Fair Value of Derivative Assets and Liabilities	The following table presents t he fair value of derivative assets and liabilities within the consolidated balance sheets as of December 31, 2023 and December 31, 2022 (in thousands): December 31, 2023 December 31, 2022 Assets Liabilities Assets Liabilities Derivatives not designated as cash flow hedging instruments: Other current assets $ 33 — — — Other non-current assets — — — — Accrued expenses and other liabilities — — — — Other non-current liabilities — $ 62 — — Derivatives designated as cash flow hedging instruments: Other current assets — — $ 5,028 — Other non-current assets — — — — Accrued expenses and other liabilities — — — — Other non-current liabilities — — — $ 73

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
Income Taxes (Tables)	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Schedule of Components of Income Tax (Benefit) Expense	The details of the Company's income tax expense (benefit) for the years ended December 31, 2023 and 2022 are as follows (in thousands): 2023 2022 Current: Federal $ — $ 6 State 87 37 Total current 87 43 Deferred: Federal 745 (37,634) State 1,736 (10,939) Total deferred 2,481 (48,573) Total income tax expense (benefit) $ 2,568 $ (48,530)
Schedule of Effective Income Tax Rate Reconciliation	The differences between the federal tax rate and the Company's effective tax rate for the years ended December 31, 2023 and 2022 are as follows (in thousands): 2023 2022 Federal income tax benefit at statutory rate $ (13,337) 21.0 % $ (113,731) 21.0 % State income tax benefit, net of federal tax benefit (5,847) 9.2 % (16,827) 3.1 % Change in state tax rate (576) 0.9 % 5 — % Share-based compensation 1,410 (2.2) % (304) 0.1 % Prior period adjustments and other 1,685 (2.7) % 471 (0.1) % Valuation allowance 20,235 (31.9) % 31,595 (5.8) % Changes in fair value of warrant liability and contingent common shares liability (494) 0.8 % (9,821) 1.8 % Goodwill impairment charges — — % 59,893 (11.1) % Other permanent differences, net (508) 0.9 % 189 — % Total income tax expense (benefit) $ 2,568 (4.0) % $ (48,530) 9.0 %
Schedule of Deferred Tax Assets and Liabilities	Deferred income taxes have been provided on temporary differences, which consist of the following at December 31, 2023 and 2022 (in thousands): 2023 2022 Deferred income tax assets: Accrued liabilities $ 8,895 $ 7,112 Provision for bad debt 12,097 11,828 Operating lease liabilities 56,661 64,288 Acquisition and transaction costs 2,699 3,186 Net operating losses 116,834 104,419 Interest expense 54,796 43,323 Other deferred tax assets 6,429 6,335 Total gross deferred income tax assets 258,411 240,491 Valuation allowance (110,142) (89,907) Total gross deferred income tax assets, net of valuation allowance 148,269 150,584 Deferred income tax liabilities: Goodwill 32,498 26,251 Trade name/trademark 66,183 66,445 Operating lease right-of-use assets 46,106 54,360 Depreciation 14,158 20,039 Deferred debt issuance costs and original issue discount 8,708 — Other deferred tax liabilities 1,983 2,375 Total gross deferred income tax liabilities 169,636 169,470 Net deferred income tax liabilities $ 21,367 $ 18,886

Leases (Tables)

Leases (Tables)	12 Months Ended
Leases (Tables)	Dec. 31, 2023
Leases [Abstract]
Schedule of Lease Cost, Supplemental Cash Flow, and Other Information Related to Leases	The components of the Company's lease costs incurred were as follows for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Lease cost Operating lease cost (1) $ 68,924 $ 69,533 Variable lease cost (2) 22,086 20,951 Total lease cost (3) $ 91,010 $ 90,484 (1) Includes ROU asset impairment charges of $2.3 million and $2.6 million for the years ended December 31, 2023 and 2022, respectively. (2) Includes short term lease costs, which are immaterial. (3) Sublease income primarily relates to subleases of certain clinic facilities to third parties, and is immaterial. Other supplemental quantitative disclosures were as follows for the periods indicated below (in thousands): Year Ended December 31, 2023 December 31, 2022 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 65,870 $ 72,440 Right-of-use assets obtained in exchange for new operating lease liabilities $ 7,293 $ 9,688 Average lease terms and discount rates as of December 31, 2023 and December 31, 2022 were as follows: December 31, 2023 December 31, 2022 Weighted-average remaining lease term: Operating leases 5.4 years 5.9 years Weighted-average discount rate: Operating leases 7.4% 6.9%
Schedule of Estimated Undiscounted Future Lease Payments	Estimated undiscounted future lease payments under non-cancellable operating leases, along with a reconciliation of the undiscounted cash flows to operating lease liabilities, respectively, at December 31, 2023 were as follows (in thousands): Year Amount (1) 2024 $ 66,641 2025 56,874 2026 49,925 2027 38,606 2028 27,185 Thereafter 50,955 Total undiscounted future cash flows 290,186 Less: Imputed Interest (53,054) Present value of future cash flows $ 237,132 Presentation on Balance Sheet: Current $ 51,530 Non-current $ 185,602 (1) Excludes $0.4 million of current portion of operating lease liabilities and $1.4 million of operating lease liabilities, respectively, reclassified as held for sale as of December 31, 2023. Refer to Note 3 - Divestitures for additional information.

Loss per Share (Tables)

Loss per Share (Tables)	12 Months Ended
Loss per Share (Tables)	Dec. 31, 2023
Earnings Per Share [Abstract]
Schedule of Calculation of Both Basic and Diluted Loss Per Share	The calculation of both basic and diluted loss per share for the periods indicated below was as follows (in thousands, except per share data): Year Ended December 31, 2023 December 31, 2022 Basic and diluted loss per share: Net loss $ (66,078) $ (493,047) Less: Net income (loss) attributable to non-controlling interests 3,717 (668) Less: Series A Senior Preferred redemption value adjustments (1) 38,958 — Less: Series A Senior Preferred cumulative dividend 23,219 17,876 Loss available to common stockholders $ (131,972) $ (510,255) Weighted average shares outstanding (2) 4,133 4,063 Basic and diluted loss per share $ (31.93) $ (125.59) (1) For the year ended December 31, 2023, the Series A Senior Preferred Stock was remeasured to its redemption value. For the year ended December 31, 2023, this adjustment included a one-time recognition of a deemed dividend primarily from the original issue discount and an incremental redemption value adjustment to reflect the carrying amount equal to what the redemption amount would be as if redemption were to occur at the end of the reporting period. Refer to Note 11 - Mezzanine and Stockholders' Equity for additional information. (2) Included within weighted average shares outstanding following the 2022 Debt Refinancing are common shares issuable upon the exercise of the Series II Warrants, as the Series II Warrants are exercisable at any time for nominal consideration. As such, the shares are considered to be outstanding for the purpose of calculating basic and diluted loss per share.
Schedule of Antidilutive Securities Excluded From Computation of Diluted Shares Outstanding	For the periods presented, basic and diluted loss per share were equal. The following number of shares issuable related to outstanding securities could potentially dilute earnings per share in the future (in thousands): Year Ended December 31, 2023 December 31, 2022 2L Notes (1) 8,625 — Series I Warrants 105 105 IPO Warrants 197 197 Restricted shares (2) 5 8 Stock options 99 106 RSUs 767 84 RSAs 1 4 Total 9,799 504 (1) Potential dilution is reflected on an if-converted basis based on the principal amount of 2L Notes as of the end of the periods presented, and Conversion Price of $12.50 per share. (2) Represents certain shares of Class A common stock legally issued, but not outstanding, as of the respective periods.

Overview of the Company (Detail

Overview of the Company (Details) $ in Millions	12 Months Ended
Overview of the Company (Details) $ in Millions	Dec. 31, 2022 USD ($)	Dec. 31, 2023 clinic state
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of stores		896
Number of states in which entity operates \| state		24
Number of stores under management service agreements		18
CARES Act, MAAPP Funds
Unusual or Infrequent Item, or Both [Line Items]
Proceeds from sale of Home Health service line \| $	$ 12.3

Basis of Presentation and Sum_4

Basis of Presentation and Summary of Significant Accounting Policies - Narrative (Details)			12 Months Ended
	Jun. 15, 2023 USD ($)	Jun. 14, 2023	Dec. 31, 2023 USD ($) segment	Dec. 31, 2022 USD ($)
Finite-Lived Intangible Assets [Line Items]
Reverse stock split, conversion ratio		0.02
Cash used in operations			$ 12,366,000	$ 65,508,000
Operating loss			27,517,000	539,734,000
Net loss			66,078,000	493,047,000
Principal payments on long-term debt			0	555,048,000
Total assets			1,003,281,000	1,078,985,000
Total liabilities			$ 878,743,000	890,198,000
Number of operating segments \| segment			1
Number of reportable segments \| segment			1
Restricted cash included within cash and cash equivalents			$ 800,000	800,000
Asset impairment charge			5,600,000	3,000,000
Amortization of debt issuance costs			1,600,000	1,700,000
Debt issuance discounts			1,300,000	1,200,000
Impairment charges			2,300,000	2,600,000
Salaries and related costs
Finite-Lived Intangible Assets [Line Items]
Advertising expense			2,900,000	3,400,000
Selling, general and administrative expenses
Finite-Lived Intangible Assets [Line Items]
Advertising expense			4,500,000	4,700,000
Variable Interest Entity, Primary Beneficiary \| RSFH Investment
Finite-Lived Intangible Assets [Line Items]
Total assets			10,600,000	10,100,000
Total liabilities			$ 4,900,000	$ 5,000,000
Delayed Draw Right \| Convertible Debt
Finite-Lived Intangible Assets [Line Items]
Debt amount	$ 25,000,000
Senior Secured Term Loan (due February 24, 2028) \| Secured Debt
Finite-Lived Intangible Assets [Line Items]
Principal payments on long-term debt	100,000,000
2L Notes \| Convertible Debt
Finite-Lived Intangible Assets [Line Items]
Proceeds from long-term debt	$ 100,000,000

Basis of Presentation and Sum_5

Basis of Presentation and Summary of Significant Accounting Policies - Property and Equipment (Details)	Dec. 31, 2023
Equipment \| Minimum
Property, Plant and Equipment [Line Items]
Useful life	3 years
Equipment \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	5 years
Furniture and fixtures \| Minimum
Property, Plant and Equipment [Line Items]
Useful life	5 years
Furniture and fixtures \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	7 years
Automobiles \| Minimum
Property, Plant and Equipment [Line Items]
Useful life	3 years
Automobiles \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	5 years
Software \| Minimum
Property, Plant and Equipment [Line Items]
Useful life	3 years
Software \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	5 years
Buildings
Property, Plant and Equipment [Line Items]
Useful life	40 years
Leasehold improvements \| Minimum
Property, Plant and Equipment [Line Items]
Useful life	5 years
Leasehold improvements \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	15 years

Basis of Presentation and Sum_6

Basis of Presentation and Summary of Significant Accounting Policies - Intangible Assets (Details)	Dec. 31, 2023
Non-compete agreements \| Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life	2 years
Non-compete agreements \| Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life	5 years
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Useful life	15 years

Divestitures - Assets and Liabi

Divestitures - Assets and Liabilities Classified as Held For Sale (Details) - Disposal Group, Held-for-sale, Not Discontinued Operations - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
2023 Clinics Held For Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accounts receivable, net	$ 0
Prepaid expenses	0
Property and equipment, net	674
Operating lease right-of-use assets	1,382
Goodwill, net	0
Other non-current assets	0
Total assets held for sale	2,056
Accounts payable	0
Accrued expenses and other liabilities	0
Current portion of operating lease liabilities	357
Operating lease liabilities	1,421
Total liabilities held for sale	$ 1,778
2022 Clinics Held for Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accounts receivable, net		$ 486
Prepaid expenses		23
Property and equipment, net		1,113
Operating lease right-of-use assets		1,929
Goodwill, net		3,192
Other non-current assets		12
Total assets held for sale		6,755
Accounts payable		22
Accrued expenses and other liabilities		201
Current portion of operating lease liabilities		685
Operating lease liabilities		1,706
Total liabilities held for sale		$ 2,614

Revenue from Contracts with C_3

Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Net revenue	$ 699,016	$ 635,671
Net patient revenue
Disaggregation of Revenue [Line Items]
Net revenue	$ 636,095	$ 575,940
Net operating revenue (as percent)	100%	100%
Net patient revenue \| Commercial
Disaggregation of Revenue [Line Items]
Net operating revenue (as percent)	58.60%	57.60%
Net patient revenue \| Government
Disaggregation of Revenue [Line Items]
Net operating revenue (as percent)	23.20%	24.20%
Net patient revenue \| Workers’ compensation
Disaggregation of Revenue [Line Items]
Net operating revenue (as percent)	11.70%	12.40%
Net patient revenue \| Other
Disaggregation of Revenue [Line Items]
Net operating revenue (as percent)	6.50%	5.80%
ATI Worksite Solutions
Disaggregation of Revenue [Line Items]
Net revenue	$ 37,219	$ 35,515
Management Service Agreements
Disaggregation of Revenue [Line Items]
Net revenue	14,831	12,857
Sports Medicine and other revenue
Disaggregation of Revenue [Line Items]
Net revenue	$ 10,871	$ 11,359

Goodwill, Trade Name and Othe_3

Goodwill, Trade Name and Other Intangible Assets - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) unit	Dec. 31, 2022 USD ($)
Indefinite-lived Intangible Assets [Line Items]
Number of reporting units \| unit	1
Goodwill impairment loss	$ 0	$ 318,900
Impairment, Intangible Asset, Indefinite-Lived (Excluding Goodwill), Statement of Income or Comprehensive Income [Extensible Enumeration]	Goodwill, intangible and other asset impairment charges
ATI trade name
Indefinite-lived Intangible Assets [Line Items]
Impairment of indefinite lived intangible assets		$ 164,400

Goodwill, Trade Name and Othe_4

Goodwill, Trade Name and Other Intangible Assets - Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 286,458
Impairment charges	0	$ (318,900)
Reclassifications to held and used	3,192
Goodwill, ending balance	$ 289,650	286,458
Accumulated goodwill impairment loss		$ 1,045,700

Goodwill, Trade Name and Othe_5

Goodwill, Trade Name and Other Intangible Assets - Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Finite-Lived Intangible Assets [Line Items]
Total trade name and other intangible assets, net	$ 245,858	$ 246,582
ATI trade name
Indefinite-lived Intangible Assets [Line Items]
Gross intangible assets	245,000	245,000
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross intangible assets:	2,395	2,395
Accumulated amortization:	(1,807)	(1,126)
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Gross intangible assets:	640	640
Accumulated amortization:	$ (370)	$ (327)

Property and Equipment - Carryi

Property and Equipment - Carrying Amount (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 342,066	$ 352,896
Accumulated depreciation and amortization	(241,644)	(229,206)
Property and equipment, net	100,422	123,690
Disposal Group, Held-for-sale, Not Discontinued Operations \| 2022 Clinics Held for Sale
Property, Plant and Equipment [Line Items]
Disposal group, including discontinued operation, property, plant and equipment		1,113
Disposal Group, Held-for-sale, Not Discontinued Operations \| 2023 Clinics Held For Sale
Property, Plant and Equipment [Line Items]
Disposal group, including discontinued operation, property, plant and equipment	674
Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	37,980	38,102
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	14,311	17,215
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	178,888	191,182
Automobiles
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4	19
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	108,749	102,651
Construction-in-progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,134	$ 3,727

Property and Equipment - Narrat

Property and Equipment - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Property and equipment, net	$ 100,422	$ 123,690
Amortization expense	36,688	39,846
Software and Software Development Costs
Property, Plant and Equipment [Line Items]
Property and equipment, net	66,100	64,300
Amortization expense	$ 7,700	$ 8,200

Property and Equipment - Deprec

Property and Equipment - Depreciation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Total depreciation expense	$ 36,688	$ 39,846
Rent, clinic supplies, contract labor and other
Property, Plant and Equipment [Line Items]
Total depreciation expense	25,179	27,429
Selling, general and administrative expenses
Property, Plant and Equipment [Line Items]
Total depreciation expense	$ 11,509	$ 12,417

Accrued Expenses and Other Li_3

Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Salaries and related costs	$ 37,630	$ 28,949
Accrued legal settlement	21,324	1,015
Credit balances due to patients and payors	7,712	6,117
Accrued professional fees	4,146	4,536
Accrued interest	4,913	762
Accrued occupancy costs	2,593	2,410
Accrued contract labor	2,255	4,483
Other payables and accrued expenses	7,862	5,400
Accrued expenses and other liabilities	88,435	$ 53,672
Estimated liability	$ 20,000

Borrowings - Long-Term Debt (De

Borrowings - Long-Term Debt (Details) - USD ($) $ in Thousands			12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Jun. 15, 2023	Jun. 14, 2023
Debt Instrument [Line Items]
Debt, gross	$ 556,310		$ 556,310
Paid-in-kind interest added during period			6,567	$ 3,481
Senior Secured Term Loan And 2022 Credit Agreement \| Secured Debt And Line Of Credit
Debt Instrument [Line Items]
Less: unamortized debt issuance costs	(7,395)	$ (11,137)	(7,395)	(11,137)
Less: unamortized original issue discount	(7,525)	(8,944)	(7,525)	(8,944)
Total debt, net	433,578	531,600	433,578	531,600
Less: current portion of long-term debt	0	0	0	0
Long-term debt, net	$ 433,578	$ 531,600	$ 433,578	$ 531,600
Senior Secured Term Loan (due February 24, 2028)
Debt Instrument [Line Items]
State interest rate (in percent)	12.70%	12.10%	12.70%	12.10%
Effective interest rate (in percent)	13.90%	13.10%	13.90%	13.10%
Senior Secured Term Loan (due February 24, 2028) \| Secured Debt
Debt Instrument [Line Items]
Debt, gross	$ 410,048	$ 503,481	$ 410,048	$ 503,481	$ 407,800	$ 507,800
Paid-in-kind interest added during period	10,000	3,500
2022 Credit Agreement \| Secured Debt And Line Of Credit
Debt Instrument [Line Items]
Total debt, net	433,578		433,578
2022 Credit Agreement \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Debt, gross	38,450	48,200	38,450	48,200
Less: unamortized debt issuance costs	$ (500)	$ (600)	$ (500)	$ (600)
Weighted average interest rate	9.50%	8.30%	9.50%	8.30%

Borrowings - Schedule of Fair V

Borrowings - Schedule of Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Convertible Debt \| 2L Notes
Debt Instrument [Line Items]
Fair value	$ 79,472	$ 0

Borrowings - Narrative (Details

Borrowings - Narrative (Details)					12 Months Ended
Borrowings - Narrative (Details)		Dec. 31, 2023 USD ($)	Jun. 15, 2023 USD ($) draw $ / shares shares	Feb. 24, 2022 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Jan. 30, 2024 USD ($)	Jun. 14, 2023 USD ($)
Debt Instrument [Line Items]
Principal payments on long-term debt					$ 0	$ 555,048,000
Debt, gross		$ 556,310,000			556,310,000
Loss on extinguishment of debt					444,000	2,809,000
Delayed draw right assets			$ 3,500,000
Warrants purchase common stock aggregate stated value						165,000,000
Long-term debt, net	[1]	433,578,000			433,578,000	531,600,000
Proceeds from revolving line of credit					35,000,000	48,200,000
Payments on revolving line of credit					44,750,000	0
Related Party
Debt Instrument [Line Items]
Long-term debt, net		17,000,000			17,000,000
Series B Preferred Stock, Voting Rights
Debt Instrument [Line Items]
Preferred stock, convertible, conversion price (in dollars per share) \| $ / shares			$ 12.87
Convertible Debt \| Knighthead Capital Management, LLC \| Subsequent Event
Debt Instrument [Line Items]
Debt amount							$ 12,000,000
Convertible Debt \| Marathon Asset Management LP \| Subsequent Event
Debt Instrument [Line Items]
Debt amount							8,000,000
Convertible Debt \| Caspian Capital L.P \| Subsequent Event
Debt Instrument [Line Items]
Debt amount							5,000,000
2L Notes \| Convertible Debt
Debt Instrument [Line Items]
Proceeds from long-term debt			$ 100,000,000
Debt, gross		107,812,000	103,243,000		107,812,000
Original issuance discount (premium)			(700,000)
Delayed draw right assets			$ 2,800,000
State interest rate (in percent)			8%
Debt instrument, convertible, conversion price ( in usd per share) \| $ / shares			$ 12.50
Number of shares issued with 1,000 of debt \| shares			0.001
Proceeds from issuance of additional long term debt			$ 3,200,000
Fair value		$ 79,472,000			$ 79,472,000	$ 0
Effective interest rate (in percent)		8%			8%
2L Notes \| Convertible Debt \| Onex Credit Partners, LLC
Debt Instrument [Line Items]
Proceeds from long-term debt			8,800,000
Debt, gross		$ 9,500,000			$ 9,500,000
2L Notes \| Convertible Debt \| Knighthead Capital Management, LLC
Debt Instrument [Line Items]
Proceeds from long-term debt			50,800,000
Debt, gross		54,700,000			54,700,000
2L Notes \| Convertible Debt \| Marathon Asset Management LP
Debt Instrument [Line Items]
Proceeds from long-term debt			40,400,000
Debt, gross		$ 43,600,000			$ 43,600,000
Senior Secured Term Loan (due February 24, 2028)
Debt Instrument [Line Items]
State interest rate (in percent)		12.70%			12.70%	12.10%
Effective interest rate (in percent)		13.90%			13.90%	13.10%
Senior Secured Term Loan (due February 24, 2028) \| Secured Debt
Debt Instrument [Line Items]
Principal payments on long-term debt			100,000,000
Debt, gross		$ 410,048,000	407,800,000		$ 410,048,000	$ 503,481,000		$ 507,800,000
Loss on extinguishment of debt					$ 400,000
Derecognition of the proportionate amount of remaining unamortized deferred financing costs and unamortized original issue discount			4,300,000
Original issuance discount (premium)			$ 1,800,000
Interest rate period increase			1%
Interest in-kind interest to pay				2%
Premium rate		0.50%		0.50%	0.50%
Senior Secured Term Loan (due February 24, 2028) \| Secured Debt \| HPS Investment Partners, LLC
Debt Instrument [Line Items]
Debt, gross			$ 391,000,000
Senior Secured Term Loan (due February 24, 2028) \| Secured Debt \| Onex Credit Partners, LLC
Debt Instrument [Line Items]
Debt, gross			16,300,000
Senior Secured Term Loan (due February 24, 2028) \| Secured Debt \| Knighthead Capital Management, LLC
Debt Instrument [Line Items]
Debt, gross			300,000
Senior Secured Term Loan (due February 24, 2028) \| Secured Debt \| Marathon Asset Management LP
Debt Instrument [Line Items]
Debt, gross			200,000
Senior Secured Term Loan (due February 24, 2028) \| Secured Debt \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Floor rate (as a percent)		1%
Basis spread on variable rate (as a percent)		7.25%
Delayed Draw Right \| Convertible Debt
Debt Instrument [Line Items]
Debt amount			$ 25,000,000
Debt instrument, time to draw			18 months
Number of draws \| draw			2
Liquidity period			6 months
Delayed Draw Right \| Convertible Debt \| Subsequent Event
Debt Instrument [Line Items]
Debt amount							$ 25,000,000
Delayed Draw Right, Draw One \| Convertible Debt
Debt Instrument [Line Items]
Debt amount			$ 12,500,000
Delayed Draw Right, Draw Two \| Convertible Debt
Debt Instrument [Line Items]
Debt amount			12,500,000
2022 Credit Agreement
Debt Instrument [Line Items]
Original issuance discount (premium)				$ 10,000,000
Debt amount				550,000,000
Debt issuance costs, gross				12,500,000
Prepayment upon insurance proceeds in excess of			10,000,000
2022 Credit Agreement \| Through first quarter of 2023
Debt Instrument [Line Items]
Minimum liquidity amount			30,000,000
2022 Credit Agreement \| Through second quarter 2023
Debt Instrument [Line Items]
Minimum liquidity amount			25,000,000
2022 Credit Agreement \| Through fourth quarter of 2023
Debt Instrument [Line Items]
Minimum liquidity amount			15,000,000
2022 Credit Agreement \| Through the fourth quarter of 2024
Debt Instrument [Line Items]
Minimum liquidity amount			$ 10,000,000
2022 Credit Agreement \| Beginning first quarter of 2025
Debt Instrument [Line Items]
Maximum debt to EBITDA ratio allowed			11
2022 Credit Agreement \| After first quarter of 2025
Debt Instrument [Line Items]
Maximum debt to EBITDA ratio allowed			7
2022 Credit Agreement \| Secured Debt
Debt Instrument [Line Items]
Debt amount				$ 500,000,000
2022 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Line of credit facility, commitment fee percentage				0.50%
2022 Credit Agreement \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Debt, gross		$ 38,450,000			$ 38,450,000	48,200,000
Interest rate period increase			1%
Maximum borrowing capacity				$ 50,000,000
Proceeds from revolving line of credit					35,000,000	$ 48,200,000
Payments on revolving line of credit					44,800,000
Borrowings outstanding		$ 38,500,000			$ 38,500,000
Weighted average interest rate		9.50%			9.50%	8.30%
Balance of unamortized issuance costs		$ 500,000			$ 500,000	$ 600,000
2022 Credit Agreement \| Line of Credit \| Revolving Credit Facility \| Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)		4.10%
2022 Credit Agreement \| Line of Credit \| Letter of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity				10,000,000
Letters of credit outstanding		$ 6,500,000			$ 6,500,000	1,800,000
2016 first lien term loan
Debt Instrument [Line Items]
Principal payments on long-term debt				$ 555,000,000
Loss on extinguishment of debt						$ 2,800,000

[1] Includes $17.0 million of principal amount of debt due to related parties as of December 31, 2023.

Borrowings - Schedule of Stock

Borrowings - Schedule of Stock Conversion (Details) - Series B Preferred Stock, Voting Rights - $ / shares	7 Months Ended
	Dec. 31, 2023	Jun. 15, 2023
Increase (Decrease) in Temporary Equity [Roll Forward]
Series B Preferred Stock, shares at Closing Date (in shares)	103,000
Increase (decrease) in shares during period (in shares)	5,000
Series B Preferred Stock, shares at end of period (in shares)	108,000
Common stock voting rights, as converted basis (in shares)	8,377,000
Preferred stock, convertible, conversion price (in dollars per share)		$ 12.87

Borrowings - Schedule of Princi

Borrowings - Schedule of Principal Amount (Details) - USD ($) $ in Thousands	7 Months Ended	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Debt Instrument Principal [Roll Forward]
Paid-in-kind interest added during period		$ 6,567	$ 3,481
2L Notes, principal amount at end of period	$ 556,310	556,310
2L Notes \| Convertible Debt
Debt Instrument Principal [Roll Forward]
2L Notes, principal amount at Closing Date	103,243
Paid-in-kind interest added during period	4,569
2L Notes, principal amount at end of period	$ 107,812	$ 107,812

Borrowings - Maturities (Detail

Borrowings - Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Jun. 15, 2023
Debt Instrument [Line Items]
2024	$ 0
2025	0
2026	0
2027	38,450
2028	517,860
Thereafter	0
Total future maturities	556,310
Unamortized original issue discount and debt issuance costs	(14,920)
2L Notes \| Convertible Debt
Debt Instrument [Line Items]
Total future maturities	$ 107,812	$ 103,243

Employee Benefit Plans - Narrat

Employee Benefit Plans - Narratives (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Retirement Benefits [Abstract]
Maximum employee contribution, percent	100%
Defined contribution plan, employer matching contribution (in percent)	50%
Defined contribution plan, employer matching contribution, percent of employees' gross pay (in percent)	6%
Contributions to the plan	$ 5,395	$ 4,933

Employee Benefit Plans (Details

Employee Benefit Plans (Details) - USD ($) $ in Thousands	12 Months Ended
Employee Benefit Plans (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Defined Contribution Plan Disclosure [Line Items]
Contributions to the plan	$ 5,395	$ 4,933
Salaries and related costs
Defined Contribution Plan Disclosure [Line Items]
Contributions to the plan	4,803	4,374
Selling, general and administrative expenses
Defined Contribution Plan Disclosure [Line Items]
Contributions to the plan	$ 592	$ 559

Share-Based Compensation - Narr

Share-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	10,639,000
Service-Base Vesting
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	241,000
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, expiration period	10 years
Stock options \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Stock options \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years
RSUs \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	1 year
RSUs \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
RSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Common stock, capital shares reserved for future issuance (in shares)	5,000
2016 Plan \| Service-Base Vesting
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 0.6	$ 1.1
Awards granted in period (in shares)	0
2016 Plan \| Service-Base Vesting \| Period from The Original Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	8 years
2016 Plan \| Service-Base Vesting \| Post-Closing
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
2021 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance ( in shares)	1,200,000
Common stock, capital shares reserved for future issuance (in shares)	200,000
Number of options granted during period	0
Share-based payment arrangement, nonvested award, option, cost not yet recognized, amount	$ 2.4
2021 Plan \| Service-Base Vesting
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	868,000
2021 Plan \| Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 1.5	1.3
Unvested, weighted-average remaining contractual term	1 year 10 months 24 days
2021 Plan \| RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 6.2	$ 4.4
Awards granted in period (in shares)	762,201	100,000
Unvested, weighted-average remaining contractual term	2 years
Granted (in dollars per share)	$ 16.58	$ 104.51
Fair value of vested	$ 4	$ 1.8
Non-vested awards, cost not yet recognized	10.9
2021 Plan \| RSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 0.4	$ 0.5
Awards granted in period (in shares)	0	0
Unvested, weighted-average remaining contractual term	6 months
Granted (in dollars per share)	$ 0
Fair value of vested	$ 0.4	$ 0.5
Non-vested awards, cost not yet recognized	$ 0.2

Share-Based Compensation - Sche

Share-Based Compensation - Schedule of Share-based Payment Arrangement, Option, Activity (Details) - 2021 Plan - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Number of Options
Outstanding, beginning balance (in shares)	106,495
Granted (in shares)	0
Adjustments (in shares)	2,676
Exercised (in shares)	0
Forfeited/Cancelled (in shares)	(9,886)
Outstanding, ending balance (in shares)	99,285	106,495
Exercisable (in shares)	32,181
Unvested (in shares)	67,104
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 92.14
Granted (in dollars per share)	0
Adjustments (in dollars per share)	87
Exercised (in dollars per share)	0
Forfeited/Cancelled (in dollars per share)	96.46
Outstanding, ending balance (in dollars per share)	91.73	$ 92.14
Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 99.93
Unvested, Weighted-Average Exercise Price (in dollars per share)	87.80
Weighted-Average Contractual Term (in years)
Outstanding	8 years 2 months 12 days	9 years 1 month 6 days
Exercisable, Weighted-Average Remaining Contractual Term	8 years 1 month 6 days
Unvested, Weighted-Average Remaining Contractual Term	8 years 2 months 12 days
Aggregate Intrinsic Value
Outstanding	$ 0	$ 0
Exercised	0
Exercisable, Aggregate Intrinsic Value	0
Unvested, Aggregate Intrinsic Value	$ 0

Share-Based Compensation - Valu

Share-Based Compensation - Valuation Assumption (Details) - 2021 Plan	12 Months Ended
	Dec. 31, 2022 $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value of options ( in dollar per share)	$ 48.79
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.74%
Term (years)	6 years 2 months 12 days
Volatility	61.19%
Expected dividend	0%

Share-Based Compensation - Acti

Share-Based Compensation - Activity Of Unvested RSUs And The Respective Weighted-Average Fair Value (Details) - RSUs - 2021 Plan - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
RSUs
Outstanding at beginning of period (in shares)	84,283
Granted (in shares)	762,201	100,000
Vested (in shares)	(39,145)
Forfeited (in shares)	(39,975)
Outstanding at ending of period (in shares)	767,364	84,283
Weighted-Average Grant Date Fair Value
Outstanding at beginning of period (in dollars per share)	$ 99.46
Granted (in dollars per share)	16.58	$ 104.51
Vested (in dollars per share)	101.33
Forfeited (in dollars per share)	30.22
Outstanding at ending of period (in dollars per share)	$ 20.56	$ 99.46

Share-Based Compensation - Ac_2

Share-Based Compensation - Activity Of Unvested RSAs And The Respective Weighted-Average Fair Value (Details) - RSAs - 2021 Plan - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
RSAs
Outstanding at beginning of period (in shares)	3,391
Granted (in shares)	0	0
Vested (in shares)	(2,233)
Forfeited (in shares)	(83)
Outstanding at ending of period (in shares)	1,075	3,391
Weighted-Average Grant Date Fair Value
Outstanding at beginning of period (in dollars per share)	$ 171
Granted (in dollars per share)	0
Vested (in dollars per share)	171
Forfeited (in usd per share)	171
Outstanding at ending of period (in dollars per share)	$ 171	$ 171

Mezzanine and Stockholders' E_3

Mezzanine and Stockholders' Equity - Narrative (Details)			10 Months Ended	12 Months Ended
Mezzanine and Stockholders' Equity - Narrative (Details)	Dec. 31, 2023 USD ($) vote $ / shares shares	Feb. 24, 2022 USD ($) segment $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2023 USD ($) vote $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Jun. 15, 2023 director
Class of Stock [Line Items]
Preferred stock, shares issued (in shares) \| shares	200,000		200,000	200,000	200,000
Preferred stock, par value (in dollars per share) \| $ / shares	$ 0.0001		$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock, stated value (in dollars per share) \| $ / shares	$ 1,249.06		$ 1,108.34	$ 1,249.06	$ 1,108.34
Warrants purchase common stock aggregate stated value					$ 165,000,000
Preferred stock, shares authorized (in shares) \| shares	1,000,000		1,000,000	1,000,000	1,000,000
Preferred stock, shares outstanding (in shares) \| shares	200,000		200,000	200,000	200,000
Proceeds from issuance of Series A Senior Preferred Stock				$ 0	$ 144,667,000
Proceeds from issuance of warrants		$ 20,333,000		0	20,333,000
Carrying value	$ 220,393,000		$ 140,340,000	$ 220,393,000	$ 140,340,000
Common stock, shares authorized (in shares) \| shares	470,000,000		470,000,000	470,000,000	470,000,000
Common stock, par value (in dollars per share) \| $ / shares	$ 0.0001		$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares Issued (in shares) \| shares	4,200,000		4,100,000	4,200,000	4,100,000
Common stock, shares outstanding (in shares) \| shares	4,000,000		4,000,000	4,000,000	4,000,000
Tax withholdings related to net share settlement of restricted stock awards (in shares) \| shares				5,254
Treasury stock (in shares) \| shares	6,794		2,000	6,794	2,000
Treasury stock, common, value	$ 219,000		$ 146,000	$ 219,000	$ 146,000
Series I Warrants
Class of Stock [Line Items]
Number of shares issuable by each warrant \| shares		100,000
Exercise price of warrant (in dollars per share) \| $ / shares		$ 150
Proceeds from issuance of warrants		$ 5,101,000
Issuance discount		$ 200,000
Series II Warrants
Class of Stock [Line Items]
Number of shares issuable by each warrant \| shares		100,000
Exercise price of warrant (in dollars per share) \| $ / shares		$ 0.50
Proceeds from issuance of warrants		$ 15,232,000
Issuance discount		$ 500,000
Series A Preferred
Class of Stock [Line Items]
Preferred stock, shares issued (in shares) \| shares	165,000	165,000	165,000	165,000	165,000
Preferred stock, par value (in dollars per share) \| $ / shares		$ 0.0001
Preferred stock, stated value (in dollars per share) \| $ / shares		$ 1,000
Preferred stock, shares authorized (in shares) \| shares		1,000,000
Preferred stock, shares outstanding (in shares) \| shares	165,000		165,000	165,000	165,000
Proceeds from issuance of Series A Senior Preferred Stock		$ 144,667,000
Issuance costs		2,880,000
Issuance discount		$ 1,447,000
Annual dividend rate		12%
Discount on dividends		1%
In-kind increasing percentage		1%
Dividend rate, occurrence, increase percent		2%
Dividends, preferred stock, paid-in-kind			$ 17,876,000	$ 23,219,000	$ 17,900,000
Accumulated paid in-kind dividends	$ 41,100,000
Aggregate stated value	206,095,000	$ 165,000,000	$ 182,876,000	206,095,000	$ 182,876,000
Number of directors equity holders can elect \| segment		1
Change in voting rights, ADBITDA threshold		$ 100,000,000
Change in voting rights, change in ownership percent		50.10%
Carrying value	$ 220,400,000			$ 220,400,000
Series A Senior Preferred Stock
Class of Stock [Line Items]
Number of additional directors electable by holders \| director						3
Number of total directors electable by holders \| director						4
Number of unaffiliated directors \| director						1
Class A Common Stock
Class of Stock [Line Items]
Common stock, shares authorized (in shares) \| shares	470,000,000			470,000,000
Common stock, par value (in dollars per share) \| $ / shares	$ 0.0001			$ 0.0001
Common stock voting rights \| vote	1			1
Common stock, shares Issued (in shares) \| shares	4,200,000			4,200,000
Common stock, shares outstanding (in shares) \| shares	4,000,000			4,000,000
Class A Common Stock \| Series I Warrants
Class of Stock [Line Items]
Exercise period		5 years
Class A Common Stock \| Series II Warrants
Class of Stock [Line Items]
Exercise period		5 years

Mezzanine and Stockholders' E_4

Mezzanine and Stockholders' Equity - Components of Proceeds Related to the Series A Senior Preferred Stock (Details) - USD ($) $ in Thousands		12 Months Ended
	Feb. 24, 2022	Dec. 31, 2023	Dec. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Gross proceeds allocated to Series A Senior Preferred Stock		$ 0	$ 144,667
Series A Preferred
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Gross proceeds allocated to Series A Senior Preferred Stock	$ 144,667
Less: original issue discount	(1,447)
Less: issuance costs	(2,880)
Net proceeds received from issuance of Series A Senior Preferred Stock	$ 140,340

Mezzanine and Stockholders' E_5

Mezzanine and Stockholders' Equity - Aggregate Stated Value Of Series A Senior Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	10 Months Ended	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Feb. 24, 2022
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares issued (in shares)	200,000	200,000	200,000
Preferred stock, shares outstanding (in shares)	200,000	200,000	200,000
Stated value (in dollars per share)	$ 1,108.34	$ 1,249.06	$ 1,108.34
Series A Preferred
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Aggregate stated value, beginning	$ 165,000	$ 182,876
Paid in-kind dividends	17,876	23,219	$ 17,900
Aggregate stated value, ending	$ 182,876	$ 206,095	$ 182,876
Preferred stock, shares issued (in shares)	165,000	165,000	165,000	165,000
Preferred stock, shares outstanding (in shares)	165,000	165,000	165,000
Stated value (in dollars per share)				$ 1,000

Mezzanine and Stockholders' E_6

Mezzanine and Stockholders' Equity Schedule Of Temporary Equity (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)
Increase (Decrease) in Temporary Equity [Roll Forward]
Carrying value, beginning of period	$ 140,340
Write off original issue discount	1,447
Write off issuance costs	2,880
Deemed dividend from discount on initial gross proceeds allocation	20,333
Paid-in-kind dividends recognized to carrying value	41,095
Redemption value adjustment	14,298
Carrying value, end of period	$ 220,393

Mezzanine and Stockholders' E_7

Mezzanine and Stockholders' Equity - Components of Proceeds Related to the Warrants (Details) - USD ($) $ in Thousands		12 Months Ended
	Feb. 24, 2022	Dec. 31, 2023	Dec. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Gross proceeds allocated to 2022 Warrants	$ 20,333	$ 0	$ 20,333
Less: original issue discount	(203)
Less: issuance costs	(405)
Net proceeds received from issuance of 2022 Warrants	19,725		$ 19,725
Series I Warrants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Gross proceeds allocated to 2022 Warrants	5,101
Less: original issue discount	(51)
Less: issuance costs	(102)
Net proceeds received from issuance of 2022 Warrants	4,948
Series II Warrants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Gross proceeds allocated to 2022 Warrants	15,232
Less: original issue discount	(152)
Less: issuance costs	(303)
Net proceeds received from issuance of 2022 Warrants	$ 14,777

Mezzanine and Stockholders' E_8

Mezzanine and Stockholders' Equity - Reserved Shares (Details) - $ / shares shares in Thousands	Dec. 31, 2023	Jun. 15, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	10,639
Series B Preferred Stock, Voting Rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred stock, convertible, conversion price (in dollars per share)		$ 12.87
2L Notes \| Convertible Debt
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Debt instrument, convertible, conversion price ( in usd per share)		$ 12.50
2L Notes
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	8,625
Shares available for grant under the 2021 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	241
Earnout Shares reserved
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	300
2022 Warrant shares reserved
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	230
IPO Warrant shares reserved
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	197
Vesting Shares reserved
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	173
Restricted shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	5
2021 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	200
2021 Plan \| Shares available for grant under the 2021 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	868

IPO Warrant Liability - Narrati

IPO Warrant Liability - Narrative (Details) - $ / shares shares in Millions		12 Months Ended
	Jun. 16, 2021	Dec. 31, 2023
Public Warrant
Class of Warrant or Right [Line Items]
Number of shares called by each warrant		0.1
Exercise price of warrant (in dollars per share)		$ 575
Expiration period		5 years
Waiting period for option exercise		30 days
Private Placement Warrant
Class of Warrant or Right [Line Items]
Number of shares called by each warrant		0.1
Waiting period for option exercise	30 days

IPO Warrant Liability - Warrant

IPO Warrant Liability - Warrant Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 98
Decrease in fair value	(95)	$ (4,243)
Fair value, end of period	3	98
Private Placement Warrant
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	29	1,305
Decrease in fair value	(28)	(1,276)
Fair value, end of period	1	29
Public Warrant
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	69	3,036
Decrease in fair value	(67)	(2,967)
Fair value, end of period	$ 2	$ 69

Contingent Common Shares Liab_3

Contingent Common Shares Liability - Narrative (Details)	Jun. 16, 2021 day tranche $ / shares shares
Earnout Shares
Derivative [Line Items]
Contingent consideration liability (in shares) \| shares	300,000
Number of tranches \| tranche	3
First Issuance, Earnout Shares
Derivative [Line Items]
Contingent consideration liability (in shares) \| shares	100,000
Threshold of trading days above stock price trigger (in days)	5
Threshold consecutive trading days (in days)	10
First Issuance, Earnout Shares \| Weighted Average
Derivative [Line Items]
Stock price trigger (in dollars per share) \| $ / shares	$ 600
Second Issuance, Earnout Shares
Derivative [Line Items]
Contingent consideration liability (in shares) \| shares	100,000
Threshold of trading days above stock price trigger (in days)	5
Threshold consecutive trading days (in days)	10
Second Issuance, Earnout Shares \| Weighted Average
Derivative [Line Items]
Stock price trigger (in dollars per share) \| $ / shares	$ 700
Third Issuance, Earnout Shares
Derivative [Line Items]
Contingent consideration liability (in shares) \| shares	100,000
Threshold of trading days above stock price trigger (in days)	5
Threshold consecutive trading days (in days)	10
Third Issuance, Earnout Shares \| Weighted Average
Derivative [Line Items]
Stock price trigger (in dollars per share) \| $ / shares	$ 800
Vesting Shares
Derivative [Line Items]
Contingent consideration liability (in shares) \| shares	200,000
Number of tranches \| tranche	3
First Issuance, Vesting Shares
Derivative [Line Items]
Contingent consideration liability (in shares) \| shares	100,000
Threshold of trading days above stock price trigger (in days)	5
Threshold consecutive trading days (in days)	10
First Issuance, Vesting Shares \| Weighted Average
Derivative [Line Items]
Stock price trigger (in dollars per share) \| $ / shares	$ 600
Second Issuance, Vesting Shares
Derivative [Line Items]
Contingent consideration liability (in shares) \| shares	100,000
Threshold of trading days above stock price trigger (in days)	5
Threshold consecutive trading days (in days)	10
Second Issuance, Vesting Shares \| Weighted Average
Derivative [Line Items]
Stock price trigger (in dollars per share) \| $ / shares	$ 700
Third Issuance, Vesting Shares
Derivative [Line Items]
Contingent consideration liability (in shares) \| shares	100,000
Threshold of trading days above stock price trigger (in days)	5
Threshold consecutive trading days (in days)	10
Third Issuance, Vesting Shares \| Weighted Average
Derivative [Line Items]
Stock price trigger (in dollars per share) \| $ / shares	$ 800

Contingent Common Shares Liab_4

Contingent Common Shares Liability - Derivatives and Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Schedule of Changes in Fair Value
Fair value, beginning of period	$ 2,835
Fair value, end of period	578	$ 2,835
Earnout Shares
Schedule of Changes in Fair Value
Fair value, beginning of period	1,800	28,800
Decrease in fair value	(1,433)	(27,000)
Fair value, end of period	367	1,800
Vesting Shares
Schedule of Changes in Fair Value
Fair value, beginning of period	1,035	16,560
Decrease in fair value	(824)	(15,525)
Fair value, end of period	$ 211	$ 1,035

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) $ in Millions	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Senior Secured Term Loan (due February 24, 2028) \| Reported Value Measurement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total debt, net	$ 395.1
Senior Secured Term Loan (due February 24, 2028) \| Estimate of Fair Value Measurement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total debt, net	$ 369
Fair Value, Inputs, Level 3 \| Dividend yield
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
IPO Warrants	0
Earnout shares and vesting shares	0	0
Fair Value, Inputs, Level 3 \| Dividend yield \| IPO
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
IPO Warrants	0
Money Market Funds \| Fair Value, Inputs, Level 1
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Cash and cash equivalent, fair value disclosure	$ 0	$ 30

Fair Value Measurements - Conve

Fair Value Measurements - Convertible Bond Valuation Model (Details) - 2L Notes - Convertible Debt	Dec. 31, 2023 $ / shares yr	Jun. 15, 2023 $ / shares yr
Risk-free interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Instrument, Measurement Input	0.0383	0.0390
Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Instrument, Measurement Input	0.4500	0.5000
Selected yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Instrument, Measurement Input	0.2050	0.2000
Expected term (years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Instrument, Measurement Input \| yr	4.7	5.3
Share price
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Instrument, Measurement Input \| $ / shares	6.14	10.21

Fair Value Measurements - Recog

Fair Value Measurements - Recognized in Change in Fair Value (Details) - 2L Notes - Convertible Debt $ in Thousands	7 Months Ended
	Dec. 31, 2023 USD ($)
Schedule of Changes in Fair Value
Fair value, beginning of period	$ 103,943
Decrease in fair value	(24,471)
Fair value, end of period	$ 79,472

Fair Value Measurements - Measu

Fair Value Measurements - Measurement Inputs (Details) - Fair Value, Inputs, Level 3	Dec. 31, 2023 $ / shares	Dec. 31, 2022 $ / shares
Risk-free interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
IPO Warrants	0.0409
Earnout Shares and Vesting Shares	0.0384	0.0388
Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
IPO Warrants	0.9810
Earnout Shares and Vesting Shares	0.7720	0.7460
Dividend yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
IPO Warrants	0
Earnout Shares and Vesting Shares	0	0
Expected term (years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
IPO Warrants, Term	2 years 6 months
Earnout Shares and Vesting Shares, Term	7 years 6 months	8 years 6 months
Share price
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
IPO Warrants	6.14
Earnout Shares and Vesting Shares	6.14	15.50

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Cash Flow Hedges Included in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Cash Flow Hedges
Beginning balance	$ 48,447	$ 511,507
Ending balance	(95,855)	48,447
Cash Flow Hedges
Cash Flow Hedges
Beginning balance	4,899	28
Unrealized gain recognized in other comprehensive income before reclassifications	801	8,310
Reclassification to interest expense, net	(5,294)	(3,439)
Ending balance	$ 406	$ 4,899

Fair Value Measurements - Sch_2

Fair Value Measurements - Schedule of Fair Value Reconciliation (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Other current assets \| Not Designated as Hedging Instrument
Derivatives designated as cash flow hedging instruments:
Assets	$ 33	$ 0
Other current assets \| Designated as Hedging Instrument
Derivatives designated as cash flow hedging instruments:
Assets	0	5,028
Other non-current assets \| Not Designated as Hedging Instrument
Derivatives designated as cash flow hedging instruments:
Assets	0	0
Other non-current assets \| Designated as Hedging Instrument
Derivatives designated as cash flow hedging instruments:
Assets	0	0
Accrued expenses and other liabilities \| Not Designated as Hedging Instrument
Derivatives designated as cash flow hedging instruments:
Liabilities	0	0
Accrued expenses and other liabilities \| Designated as Hedging Instrument
Derivatives designated as cash flow hedging instruments:
Liabilities	0	0
Other non-current liabilities \| Not Designated as Hedging Instrument
Derivatives designated as cash flow hedging instruments:
Liabilities	62	0
Other non-current liabilities \| Designated as Hedging Instrument
Derivatives designated as cash flow hedging instruments:
Liabilities	$ 0	$ 73

Income Taxes - Summary of Compa

Income Taxes - Summary of Company's Provision for Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Current:
Federal	$ 0	$ 6
State	87	37
Total current	87	43
Deferred:
Federal	745	(37,634)
State	1,736	(10,939)
Total deferred	2,481	(48,573)
Total income tax expense (benefit)	$ 2,568	$ (48,530)

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
Income Taxes - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Income Tax Examination [Line Items]
Effect income tax rate expense (benefit)	(4.00%)	9%
Income tax expense (benefit)	$ 2,568	$ (48,530)
Federal
Income Tax Examination [Line Items]
Operating loss carryforwards	77,800	68,900
Valuation allowance period increase	13,400	25,800
State
Income Tax Examination [Line Items]
Operating loss carryforwards	39,100	35,500
Valuation allowance period increase	$ 6,800	$ 5,800

Income Taxes - Differences Betw

Income Taxes - Differences Between The Federal Tax Rate And The Company's Effective Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Federal income tax benefit at statutory rate	$ (13,337)	$ (113,731)
State income tax benefit, net of federal tax benefit	(5,847)	(16,827)
Change in state tax rate	(576)	5
Share-based compensation	1,410	(304)
Prior period adjustments and other	1,685	471
Valuation allowance	20,235	31,595
Changes in fair value of warrant liability and contingent common shares liability	(494)	(9,821)
Goodwill impairment charges	0	59,893
Other permanent differences, net	(508)	189
Total income tax expense (benefit)	$ 2,568	$ (48,530)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal income tax benefit at statutory rate	21%	21%
State income tax benefit, net of federal tax benefit	9.20%	3.10%
Change in state tax rate	0.90%	0%
Share-based compensation	(2.20%)	0.10%
Prior period adjustments and other	(2.70%)	(0.10%)
Valuation allowance	(31.90%)	(5.80%)
Changes in fair value of warrant liability and contingent common shares liability	0.80%	1.80%
Goodwill impairment charges	0%	(11.10%)
Other permanent differences, net	0.90%	0%
Total income tax expense (benefit)	(4.00%)	9%

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred income tax assets:
Accrued liabilities	$ 8,895	$ 7,112
Provision for bad debt	12,097	11,828
Operating lease liabilities	56,661	64,288
Acquisition and transaction costs	2,699	3,186
Net operating losses	116,834	104,419
Interest expense	54,796	43,323
Other deferred tax assets	6,429	6,335
Total gross deferred income tax assets	258,411	240,491
Valuation allowance	(110,142)	(89,907)
Total gross deferred income tax assets, net of valuation allowance	148,269	150,584
Deferred income tax liabilities:
Goodwill	32,498	26,251
Trade name/trademark	66,183	66,445
Operating lease right-of-use assets	46,106	54,360
Depreciation	14,158	20,039
Deferred debt issuance costs and original issue discount	8,708	0
Other deferred tax liabilities	1,983	2,375
Total gross deferred income tax liabilities	169,636	169,470
Net deferred income tax liabilities	$ 21,367	$ 18,886

Leases - Narrative (Details)

Leases - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Leases - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Lessee, Lease, Description [Line Items]
Operating lease assets additions	$ 12.1	$ 13.7
Operating lease liabilities, additions	$ 12.1	$ 13.7
Minimum
Lessee, Lease, Description [Line Items]
Initial operating lease term	7 years
Maximum
Lessee, Lease, Description [Line Items]
Initial operating lease term	10 years

Leases - Lease Cost (Details)

Leases - Lease Cost (Details) - USD ($) $ in Thousands	12 Months Ended
Leases - Lease Cost (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Leases [Abstract]
Operating lease cost	$ 68,924	$ 69,533
Variable lease cost	22,086	20,951
Total lease cost	91,010	90,484
Impairment charges	$ 2,300	$ 2,600

Leases - Supplemental Cash Flow

Leases - Supplemental Cash Flow (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 65,870	$ 72,440
Right-of-use assets obtained in exchange for new operating lease liabilities	$ 7,293	$ 9,688

Leases - Other Information (Det

Leases - Other Information (Details)	Dec. 31, 2023	Dec. 31, 2022
Leases [Abstract]
Weighted-average remaining lease term: Operating leases	5 years 4 months 24 days	5 years 10 months 24 days
Weighted-average discount rate: Operating leases	7.40%	6.90%

Leases - Maturity (Details)

Leases - Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
2024	$ 66,641
2025	56,874
2026	49,925
2027	38,606
2028	27,185
Thereafter	50,955
Total undiscounted future cash flows	290,186
Less: Imputed Interest	(53,054)
Present value of future cash flows	237,132
Presentation on Balance Sheet:
Current	51,530	$ 47,676
Non-current	185,602	$ 218,424
Disposal Group, Held-for-sale, Not Discontinued Operations \| 2023 Clinics Held For Sale
Presentation on Balance Sheet:
Current portion of operating lease liabilities	357
Operating lease liabilities	$ 1,421

Commitments and Contingencies (

Commitments and Contingencies (Details) $ in Thousands		10 Months Ended	12 Months Ended
Commitments and Contingencies (Details) $ in Thousands	Aug. 16, 2021 plaintiff	Sep. 22, 2022 plaintiff	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Jan. 16, 2024 USD ($)	Nov. 21, 2022 segment
Loss Contingencies [Line Items]
Contractual obligation			$ 11,600
Estimated liability			20,000
Insurance recovery receivable			23,981	$ 933
Number of plaintiffs who filed consolidated amended complaint \| segment						4
Insurance recoveries			7,900
Proceeds from legal cost insurance reimbursements			4,700
Subsequent Event
Loss Contingencies [Line Items]
Contractual obligation					$ 27,000
Payor Dispute
Loss Contingencies [Line Items]
Loss on litigation settlement				$ 3,000
ATI Shareholders vs ATI Individual Defendants
Loss Contingencies [Line Items]
Number of plaintiffs \| plaintiff	2
Insurance recovery receivable			$ 20,000
Derivative Action
Loss Contingencies [Line Items]
Number of plaintiffs \| plaintiff		5
Claims filed \| plaintiff		4

Loss per Share - Loss per Share

Loss per Share - Loss per Share Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Basic and diluted loss per share:
Net loss	$ (66,078)	$ (493,047)
Less: Net income (loss) attributable to non-controlling interests	3,717	(668)
Less: Series A Senior Preferred redemption value adjustments	38,958	0
Less: Series A Senior Preferred Stock cumulative dividend	23,219	17,876
Loss available to common stockholders, basic	(131,972)	(510,255)
Loss available to common stockholders, diluted	$ (131,972)	$ (510,255)
Weighted average shares outstanding, basic (in shares)	4,133	4,063
Weighted average shares outstanding, diluted (in shares)	4,133	4,063
Basic loss per share (in dollars per share)	$ (31.93)	$ (125.59)
Diluted loss per share (in dollars per share)	$ (31.93)	$ (125.59)

Loss per Share - Antidilutive S

Loss per Share - Antidilutive Securities (Details) - $ / shares		12 Months Ended
	Jun. 16, 2021	Dec. 31, 2023	Dec. 31, 2022	Jun. 15, 2023
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities (in shares)		9,799,000	504,000
2L Notes \| Convertible Debt
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Debt instrument, convertible, conversion price ( in usd per share)				$ 12.50
Earnout Shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Contingent consideration liability (in shares)	300,000
Vesting Shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Contingent consideration liability (in shares)	200,000
2L Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities (in shares)		8,625,000	0
Series I Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities (in shares)		105,000	105,000
IPO Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities (in shares)		197,000	197,000
Restricted shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities (in shares)		1,000	4,000
Restricted shares \| Wilco Holdco, Inc.
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities (in shares)		5,000	8,000
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities (in shares)		99,000	106,000
RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities (in shares)		767,000	84,000