Wayfair (W) 10-K28 Feb 202019 FY Annual reportFinancial data

Company Profile

Cover

Cover - USD ($) $ in Billions	12 Months Ended
Cover - USD ($) $ in Billions	Dec. 31, 2019	Feb. 18, 2020	Jun. 30, 2019
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity File Number	001-36666
Entity Registrant Name	Wayfair Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	36-4791999
Entity Address, Address Line One	4 Copley Place
Entity Address, City or Town	Boston,
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02116
City Area Code	617
Local Phone Number	532-6100
Title of 12(b) Security	Class A Common Stock, $0.001 par value
Trading Symbol	W
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 9.1
Documents Incorporated by Reference	Certain sections of the registrant's definitive Proxy Statement for the 2020 Annual Meeting of Stockholders to be filed with the Securities and Exchange Commission pursuant to Rule 14A not later than 120 days after end of this fiscal year covered by this Form 10-K are incorporated by reference into Part III of this Form 10-K.
Entity Central Index Key	0001616707
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Class A common stock
Entity Common Stock, Shares Outstanding		67,164,350
Class B common stock
Entity Common Stock, Shares Outstanding		26,958,041

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 582,753	$ 849,461
Short-term investments	404,252	114,278
Accounts receivable, net of allowance of $22,774 and $9,312 at December 31, 2019 and 2018, respectively	99,720	50,603
Inventories	61,692	46,164
Prepaid expenses and other current assets	228,721	195,430
Total current assets	1,377,138	1,255,936
Operating lease right-of-use assets	763,400
Property and equipment, net	624,544	606,977
Goodwill and intangible assets, net	18,809	2,585
Long-term investments	155,690	6,526
Other noncurrent assets	13,467	18,826
Total assets	2,953,048	1,890,850
Current liabilities
Accounts payable	908,097	650,174
Accrued expenses	298,918	212,997
Unearned revenue	167,641	148,057
Other current liabilities	236,863	127,995
Total current liabilities	1,611,519	1,139,223
Long-term debt	1,456,195	738,904
Operating lease liabilities	822,602
Lease financing obligations, net of current portion	0
Lease financing obligations, net of current portion		183,056
Other liabilities	6,940	160,388
Total liabilities	3,897,256	2,221,571
Commitments and contingencies (Note 8)
Convertible preferred stock, $0.001 par value per share: 10,000,000 shares authorized and none issued at December 31, 2019 and 2018	0	0
Stockholders’ deficit:
Additional paid-in capital	1,122,548	753,657
Accumulated deficit	(2,065,423)	(1,082,689)
Accumulated other comprehensive (loss)	(1,427)	(1,780)
Total stockholders' deficit	(944,208)	(330,721)
Total liabilities and stockholders' deficit	2,953,048	1,890,850
Class A common stock
Stockholders’ deficit:
Common stock	67	63
Class B common stock
Stockholders’ deficit:
Common stock	$ 27	$ 28

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounts receivable allowance	$ 22,774	$ 9,312
Convertible redeemable preferred units, par value (in dollars per share)	$ 0.001	$ 0.001
Convertible redeemable preferred units, shares authorized (in shares)	10,000,000	10,000,000
Convertible redeemable preferred units, shares issued (in shares)	0	0
Class A common stock
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	500,000,000	500,000,000
Common stock, shares issued (in shares)	66,642,611	62,329,701
Common stock, shares outstanding (in shares)	66,642,611	62,329,701
Class B common stock
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	164,000,000	164,000,000
Common stock, shares issued (in shares)	26,957,815	28,417,882
Common stock, shares outstanding (in shares)	26,957,815	28,417,882

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Net revenue	$ 9,127,057	$ 6,779,174	$ 4,720,895
Cost of goods sold	6,979,725	5,192,451	3,602,072
Gross profit	2,147,332	1,586,723	1,118,823
Operating expenses:
Customer service and merchant fees	356,727	260,046	169,516
Advertising	1,095,840	774,189	549,959
Selling, operations, technology, general and administrative	1,624,706	1,025,767	634,801
Total operating expenses	3,077,273	2,060,002	1,354,276
Loss from operations	(929,941)	(473,279)	(235,453)
Interest (expense), net	(54,514)	(28,560)	(9,433)
Other income (expense), net	2,881	(204)	758
Loss before income taxes	(981,574)	(502,043)	(244,128)
Provision for income taxes, net	3,010	2,037	486
Net loss	$ (984,584)	$ (504,080)	$ (244,614)
Net loss per share, basic and diluted (in dollars per share)	$ (10.68)	$ (5.63)	$ (2.81)
Weighted average number of common stock outstanding used in computing per share amounts, basic and diluted (in shares)	92,200	89,472	86,983

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net loss	$ (984,584)	$ (504,080)	$ (244,614)
Other comprehensive loss:
Foreign currency translation adjustments	120	553	(2,196)
Net unrealized gain (loss) on available-for-sale investments	233	30	(180)
Comprehensive loss	$ (984,231)	$ (503,497)	$ (246,990)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) shares in Thousands, $ in Thousands	Total	Class A and Class B Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive (Loss) Income
Beginning balance (in shares) at Dec. 31, 2016		85,831
Beginning balance at Dec. 31, 2016	$ 79,384	$ 86	$ 409,225	$ (329,940)	$ 13
Increase (Decrease) in Stockholders' Equity
Net loss	(244,614)			(244,614)
Other comprehensive income	(2,376)				(2,376)
Exercise of options to purchase common stock (in shares)		84
Exercise of options to purchase common stock	244		244
Issuance of common stock upon vesting of RSUs (in shares)		2,327
Issuance of common stock upon vesting of RSUs	2	$ 2
Shares withheld related to net settlement of RSUs (in shares)		(33)
Shares withheld related to net settlement of RSUs	(1,562)		(1,562)
Equity-based compensation expense	71,380		71,380
Equity component of issuance of convertible notes and capped calls, net (Note 15)	49,213		49,213
Ending balance (in shares) at Dec. 31, 2017		88,209
Ending balance at Dec. 31, 2017	(48,329)	$ 88	537,212	(583,266)	(2,363)
Increase (Decrease) in Stockholders' Equity
Net loss	(504,080)			(504,080)
Other comprehensive income	583				583
Exercise of options to purchase common stock (in shares)		46
Exercise of options to purchase common stock	138		138
Issuance of common stock upon vesting of RSUs (in shares)		2,504
Issuance of common stock upon vesting of RSUs	3	$ 3
Shares withheld related to net settlement of RSUs (in shares)		(11)
Shares withheld related to net settlement of RSUs	(1,284)		(1,284)
Equity-based compensation expense	133,638		133,638
Equity component of issuance of convertible notes and capped calls, net (Note 15)	83,953		83,953
Ending balance (in shares) at Dec. 31, 2018		90,748
Ending balance at Dec. 31, 2018	(330,721)	$ 91	753,657	(1,082,689)	(1,780)
Increase (Decrease) in Stockholders' Equity
Net loss	(984,584)			(984,584)
Other comprehensive income	353				353
Exercise of options to purchase common stock (in shares)		35
Exercise of options to purchase common stock	113		113
Issuance of common stock upon vesting of RSUs (in shares)		2,836
Issuance of common stock upon vesting of RSUs	3	$ 3
Shares withheld related to net settlement of RSUs (in shares)		(19)
Shares withheld related to net settlement of RSUs	(2,236)		(2,236)
Equity-based compensation expense	240,448		240,448
Equity component of issuance of convertible notes and capped calls, net (Note 15)	130,566		130,566
Ending balance (in shares) at Dec. 31, 2019		93,600
Ending balance at Dec. 31, 2019	$ (944,208)	$ 94	$ 1,122,548	$ (2,065,423)	$ (1,427)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (984,584)	$ (504,080)	$ (244,614)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	192,419	123,542	87,020
Equity-based compensation	227,451	127,564	67,840
Amortization of discount and issuance costs on convertible notes	62,111	22,585	5,830
Other non-cash adjustments	(1,691)	(56)	1,198
Changes in operating assets and liabilities:
Accounts receivable	(49,187)	(12,792)	(18,172)
Inventories	(15,631)	(18,319)	(9,454)
Prepaid expenses and other current assets	(32,590)	(65,195)	(39,124)
Accounts payable and accrued expenses	330,325	285,064	104,184
Unearned revenue and other liabilities	75,888	134,705	81,354
Other assets	(1,329)	(8,157)	(2,428)
Net cash (used in) provided by operating activities	(196,818)	84,861	33,634
Cash flows from investing activities:
Purchase of short- and long-term investments	(553,858)	(99,002)	(54,551)
Sale and maturities of short-term investments	115,468	61,068	71,095
Purchase of property and equipment	(271,742)	(159,205)	(100,451)
Site and software development costs	(129,138)	(62,750)	(46,428)
Other investing activities, net	(15,567)	(398)	0
Net cash used in investing activities	(854,837)	(260,287)	(130,335)
Cash flows from financing activities:
Proceeds from issuance of convertible notes, net of issuance costs	935,146	562,047	420,449
Premiums paid for capped call confirmations	(145,728)	(93,438)	(44,160)
Taxes paid related to net share settlement of equity awards	(2,236)	(1,284)	(1,562)
Deferred financing costs	(791)	0	0
Net proceeds from exercise of stock options	113	138	244
Net cash provided by financing activities	786,504	467,463	374,971
Effect of exchange rate changes on cash and cash equivalents	(1,557)	(1,536)	850
Net (decrease) increase in cash and cash equivalents	(266,708)	290,501	279,120
Cash and cash equivalents:
Beginning of year	849,461	558,960	279,840
End of year	582,753	849,461	558,960
Supplemental Cash Flow Information:
Cash paid for interest on long-term debt	7,763	1,554	0
Cash paid for interest on finance lease obligations	0	9,058	0
Purchase of property and equipment included in accounts payable and accrued expenses and in other liabilities	41,181	15,383	8,533
Construction costs capitalized under finance lease obligations and other leases	$ 0	$ 125,796	$ 47,276

Description of Business

Description of Business	12 Months Ended
Description of Business	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	1. Description of Business Wayfair is one of the world's largest online destinations for the home. Through its e-commerce business model, the Company offers visually inspired browsing, compelling merchandising, easy product discovery and attractive prices for over eighteen million products from over 12,000 suppliers.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
Summary of Significant Accounting Policies	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying Consolidated Financial Statements have been prepared in conformity with accounting principles generally accepted in the United States of America ("GAAP"). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification ("ASC") and Accounting Standards Update ("ASU") of the Financial Accounting Standards Board ("FASB"). Principles of Consolidation The accompanying Consolidated Financial Statements of Wayfair Inc. include its wholly-owned subsidiaries (collectively the "Company" or "Wayfair"). All intercompany accounts and transactions have been eliminated. Below is a summary of the wholly-owned subsidiaries of the Company with operations: Subsidiary Location Wayfair LLC U.S. CastleGate Logistics Inc. U.S. SK Retail, Inc. U.S. Wayfair Maine LLC U.S. Wayfair Transportation LLC U.S. Wayfair Securities Corporation U.S. Wayfair Stores Limited Republic of Ireland Wayfair (UK) Limited United Kingdom Wayfair GmbH Germany CastleGate Logistics Canada Inc. Canada CastleGate Logistics Hong Kong Limited Hong Kong Wayfair (BVI) Ltd. British Virgin Islands Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities, at the date of the Consolidated Financial Statements , and the reported amounts of revenue and expenses during the reported period. Actual results could differ from those estimates. Cash and Cash Equivalents The Company considers all highly liquid investments purchased with an original maturity (at the date of purchase) of three months or less to be the equivalent of cash for the purpose of Consolidated Balance Sheets and Statements of Cash Flows presentation. Cash equivalents, which consist primarily of money market accounts, are carried at cost, which approximates market value. Short-Term Investments and Marketable Securities Short-term investments consist of certificates of deposits and marketable securities with original maturities of greater than three months and maturing in less than twelve months from the balance sheet date. The Company classifies its marketable securities as "available-for-sale" securities. Available-for-sale securities are classified as short- and long-term investments on the Consolidated Balance Sheets and are carried at fair value. Unrealized gains and losses on available-for-sale securities that are considered temporary are recorded, net of taxes, in the accumulated other comprehensive (loss) caption of the Company’s Consolidated Balance Sheets . Unrealized losses, excluding losses related to the credit rating of the security (credit losses), on available-for-sale securities that are considered other-than-temporary but relate to securities that the Company (i) does not intend to sell and (ii) will not be required to sell below cost are also recorded, net of taxes, in accumulated other comprehensive (loss) . Further, the Company does not believe it will be required to sell such securities below cost. Therefore, the only other-than-temporary losses the Company records in other (income) expense, net in its Consolidated Statements of Operations are related to credit losses. As of December 31, 2019 and 2018 , the Company’s available-for-sale securities consisted of corporate bonds and other government obligations that are priced at fair value. The maturities of the Company’s long-term marketable securities generally range from one to two years . The cost basis of a marketable security sold is determined by the Company using the specific identification method. Fair Value of Financial Instruments The Company's financial assets and liabilities are measured at fair value, which is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date (exit price). The three levels of inputs used to measure fair value are as follows: • Level 1—Unadjusted quoted prices in active markets for identical assets or liabilities • Level 2—Unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or inputs other than quoted prices that are observable or can be corroborated by observable market data for substantially the full-term of the asset or liability • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the asset or liability This hierarchy requires the Company to use observable market data, when available, and to minimize the use of unobservable inputs when determining fair value. The Company measures its cash equivalents and short- and long-term investments at fair value. The Company classifies its cash equivalents within Level 1 because the fair value of the financial asset is valued using quoted market prices of an identical asset. The Company classifies short- and long-term investments within Level 2 because unadjusted quoted prices for identical or similar assets in markets are not active. The Company does not have any assets or liabilities classified as Level 3 financial assets. Refer to Note 3 , Marketable Securities and Fair Value Measurements , for additional detail. Concentrations of Credit Risk Financial instruments that subject the Company to credit risk consist of cash and cash equivalents, short- and long-term investments, and accounts receivable. The risk with respect to cash and cash equivalents is minimized by the Company's policy to maintain these balances with major financial institutions of high-credit quality. At times, cash balances may exceed federally insured limits; however, to date, the Company has not incurred any losses on these investments. As of December 31, 2019 and 2018 , the Company had $48.2 million and $129.5 million , respectively, in banks located outside the U.S. The risk with respect to short- and long-term investments is minimized by the Company's policy of investing in financial instruments issued by highly-rated financial institutions. The risk with respect to accounts receivable is managed by the Company through its policy of monitoring the creditworthiness of its customers to which it grants credit terms in the normal course of business. Accounts Receivable Accounts receivable are stated net of an allowance for doubtful accounts, which is based on historical losses, existing economic conditions, and other information available at the Consolidated Balance Sheets dates. Uncollectible amounts are written off against the allowance after all collection efforts have been exhausted. Inventories Inventories consisting of finished goods are stated at the lower of cost or net realizable value, determined by the first-in, first-out (FIFO) method, and consist of product for resale. Inventory costs consist of cost of product and inbound shipping and handling costs. Inventory costs also include direct and indirect labor costs, rents and depreciation expenses associated with the Company's fulfillment centers. Inventory valuation requires the Company to make judgments, based on currently available information, about the likely method of disposition, such as through sales to individual customers, liquidations, and expected recoverable values of each disposition category. Goods In-Transit Goods in-transit to customers are recorded in prepaid expenses and other current assets in the Consolidated Balance Sheets . Risk of loss and the transfer of title from the supplier to the Company occur at freight on board shipping point. As of December 31, 2019 and 2018 , goods in-transit amounted to $104.9 million and $83.7 million , respectively. Property and Equipment Property and equipment are stated at cost, net of depreciation and amortization . Expenditures for maintenance and repairs are charged to expense as incurred, whereas betterments are capitalized as additions to property and equipment. Depreciation and amortization on property and equipment is calculated on the straight-line method over the estimated useful lives of the assets as follows: Class Range of Life (In Years) Furniture and computer equipment 3 to 7 Site and software development costs 2 Leasehold improvements The lesser of useful life or lease term Buildings (leased - Note 6) 30 Site and Software Development Costs The Company capitalizes certain costs associated with the development of its sites and internal-use software products after the preliminary project stage is complete and until the software is ready for its intended use. The capitalized costs are amortized over a two-year period. Costs incurred in the preliminary stages of development, after the software is ready for its intended use and for maintenance of internal-use software are expensed as incurred. Upgrade and enhancements are capitalized to the extent they will result in added functionality. Total costs capitalized, net of accumulated amortization, totaled $123.5 million and $62.9 million as of December 31, 2019 and 2018 , respectively, and are included in property and equipment, net in the Consolidated Balance Sheets . Amortization expense for the years ended December 31, 2019 , 2018 , and 2017 were $81.6 million , $51.3 million , and $34.5 million , respectively. Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances, such as service discontinuance or technological obsolescence, indicate that the carrying amount of the long-lived asset may not be recoverable. When such events occur, the Company compares the carrying amount of the asset to the undiscounted expected future cash flows related to the asset. If the comparison indicates that an impairment exists, the amount of the impairment is calculated as the difference between the excess of the carrying amount over the fair value of the asset. If a readily determinable market price does not exist, fair value is estimated using discounted expected cash flows attributable to the asset. For the years ended December 31, 2019 , 2018 , and 2017 , no impairment of long-lived assets or identifiable intangibles had been indicated. Leases The Company adopted ASU No. 2016-02, "Leases" (ASU 2016-02) on January 1, 2019 using the modified retrospective approach. The Company also elected the package of practical expedients, which among other things, allowed the Company to carryforward historical lease classification. The adoption of the standard resulted in (1) the derecognition of building assets and finance lease obligations for certain leases that did not pass the sale-leaseback criteria, (2) the derecognition of construction in progress assets and other long-term liabilities for certain lease arrangements whereby the Company is no longer considered the deemed construction owner of the construction projects, and (3) the recognition of operating lease right-of-use ("ROU") assets and lease liabilities for lease arrangements with an initial term greater than twelve months. Adoption of ASU 2016-02 resulted in the following adjustments to the Consolidated Balance Sheet: December 31, 2018 ASU 2016-02 Adjustments January 1, 2019 (in thousands) Balance sheet line item: Property and equipment, net $ 606,977 $ (227,709 ) $ 379,268 Operating lease right-of-use assets $ — $ 524,082 $ 524,082 Other noncurrent assets $ 18,826 $ (6,814 ) $ 12,012 Other liabilities $ 160,388 $ (155,996 ) $ 4,392 Other current liabilities $ 127,995 $ (9,624 ) $ 118,371 Lease financing obligations, net of current portion $ 183,056 $ (183,056 ) $ — Operating lease liabilities $ — $ 636,385 $ 636,385 Accumulated deficit $ (1,082,689 ) $ 1,850 $ (1,080,839 ) The adoption of the standard did not have a notable impact on the Company's liquidity or a materially adverse impact on the Company's debt covenant compliance. The Company determines if an arrangement is a lease at inception. Operating leases are included in operating lease right-of-use assets, other current liabilities, and operating lease liabilities on our Consolidated Balance Sheets . As of December 31, 2019 , the Company has not entered into material financing lease arrangements. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. The Company's lease terms may include options to extend or terminate the lease when it is reasonably certain that management will exercise that option. The Company has lease arrangements with lease and non-lease components. For the Company's warehouse and fulfillment center lease arrangements, the Company accounts for lease and non-lease components separately. For all other lease arrangements, the Company accounts for lease and non-lease components as a single lease component. Operating lease ROU assets and lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at commencement date. As most of the Company's leases do not provide an implicit rate, the Company uses its estimated incremental borrowing rate ("IBR") based on the information available at commencement date in determining the present value of future payments. The determination of the Company’s IBR requires judgment. The IBR for each lease is primarily based on publicly-available information for companies within the same industry and with similar credit profiles. The rate is then adjusted for the impact of collateralization, the lease term, and other specific terms included in the Company’s lease arrangements. The IBR was determined at lease commencement, or as of January 1, 2019 for operating leases existing upon adoption of ASC 842. The IBR is subsequently reassessed upon a modification to the lease arrangement. The operating lease ROU asset also includes any lease payments made prior to commencement date and excludes lease incentives and initial direct costs incurred. ROU assets are subsequently assessed for impairment in accordance with the Company’s accounting policy for long-lived assets. Foreign Currency Translation The functional currency of the Company is the U.S. dollar, while the functional currency of certain wholly-owned subsidiaries outside of the U.S. is as follows: Subsidiary Currency Wayfair Stores Limited Euro Wayfair GmbH Euro Wayfair (BVI) Ltd. Euro Wayfair (UK) Limited Pound sterling CastleGate Logistics Canada Inc. Canadian dollar CastleGate Logistics Hong Kong Limited Hong Kong dollar The financial statements of the Company are translated to U.S. dollars using year-end exchange rates as to assets and liabilities and average exchange rates as to revenue and expenses. Capital accounts are translated at their historical exchange rates when the capital transaction occurred. The effects of foreign currency translation are included in other comprehensive loss in the Consolidated Statements of Comprehensive Loss . Transaction gains and losses are included in the Company's Consolidated Statements of Operations . Translation adjustments arising from the use of differing exchange rates from period to period are included in accumulated other comprehensive loss within total stockholders' equity (deficit). Contingent Liabilities The Company has certain contingent liabilities that arise in the ordinary course of business activities. The Company accrues for loss contingencies when losses become probable and are reasonably estimable. If the reasonable estimate of the loss is a range and no amount within the range is a better estimate, the minimum amount of the range is recorded as a liability. The Company does not accrue for contingent losses that, in its judgment, are considered to be reasonably possible, but not probable; however, it discloses the range of such reasonably possible losses. Revenue Recognition The Company primarily generated revenue through product sales on its family of sites and through (i) product sales on websites operated by third parties and (ii) fees earned for media solutions. The Company recognizes net revenue on product sales through the Company's family of sites and third party operated websites using the gross method when the Company has concluded it controls the product before it is transferred to the customer. The Company controls products when it is the entity responsible for fulfilling the promise to the customer and takes responsibility for the acceptability of the goods, assumes inventory risk from shipment through the delivery date, has discretion in establishing prices, and selects the suppliers of products sold. The Company recognizes net revenue from sales of its products upon delivery to the customer. As the Company ships a large volume of packages through multiple carriers, actual delivery dates may not always be available and as such the Company estimates delivery dates based on historical data. Net revenue from product sales includes shipping costs charged to the customer and is recorded net of taxes collected from customers, which are remitted to governmental authorities. Cash discounts and rebates earned by customers at the time of purchase are deducted from gross revenue in determining net revenue. Allowances for sales returns are estimated and recorded based on prior returns history, recent trends, and projections for returns on sales in the current period. Allowances for sales returns at December 31, 2019 and 2018 , were $ 38.0 million and $35.7 million , respectively. Actual returns in subsequent periods have been consistent with estimated amounts. The Company also earns revenue through third-party advertisers that pay based on the number of advertisement related clicks, actions, or impressions for advertisements placed on the Company's sites. Revenue earned under these arrangements is included in net revenue and is recognized in the period in which the click, action, or impression occurs. Net revenue from contracts with customers is disaggregated by geographic region because this manner of disaggregation best depicts how the nature, amount, timing, and uncertainty of revenue and cash flows are affected by economic factors. Refer to Note 12 , Segment and Geographic Information , for additional detail. The Company has three types of contractual liabilities: (i) cash collections from its customers prior to delivery of products purchased, which are initially recorded in unearned revenue , and are recognized as net revenue when the products are delivered, (ii) unredeemed gift cards and site credits, which are initially recorded in unearned revenue, and are recognized in the period they are redeemed, and (iii) membership rewards redeemable for future purchases, which are earned by customers on purchases made with the Company's Wayfair branded, private label credit card, and are initially recorded in other current liabilities, and are recognized as net revenue when redeemed. The portion of gift cards and store credits not expected to be redeemed are recognized as net revenue based on historical redemption pattern which is substantially within twenty-four months from the date of issuance. Contractual liabilities included in unearned revenue and other current liabilities in the Consolidated Balance Sheets were $167.6 million and $4.6 million at December 31, 2019 and $148.1 million and $3.1 million at December 31, 2018 , respectively. During the year ended December 31, 2019 , the Company recognized $126.0 million and $3.1 million of net revenue included in unearned revenue and other current liabilities, respectively, which was recorded as of December 31, 2018 in the Consolidated Balance Sheets . The Company adopted ASU 2014-09 as of January 1, 2018 using a modified retrospective approach and recognized a $4.7 million cumulative-effect adjustment to reduced accumulated deficit. The cumulative-effect adjustment to accumulated deficit was due to breakage of gift cards and site credits, to the extent there is no requirement for remitting balances to governmental agencies. Prior period balances were not retrospectively adjusted. The adoption of ASU 2014-09 did not have a material impact on the Company's Consolidated Balance Sheet and financial results for the year ended December 31, 2018 . Cost of Goods Sold Cost of goods sold consist of: Product Costs: The Company recognizes costs related to the purchase price of products sold, expenses capitalized into Wayfair inventory, which include direct and indirect labor costs, rent, and depreciation expenses, and inbound shipping and handling costs for Wayfair inventory. Shipping and Fulfillment Costs: Shipping costs include outbound shipping costs. Fulfillment costs include costs incurred to operate and staff our fulfillment centers and provide other inbound supply chain services such as ocean freight and drayage. Costs to operate and staff the CastleGate and WDN networks include rent and depreciation expenses associated with various facilities, costs to receive, inspect, pick, package and prepare customer orders for delivery, and direct and indirect labor costs including payroll, payroll-related benefits, and equity-based compensation. Vendor Rebates The Company earns rebates on incentive programs with its suppliers. These rebates are earned upon shipment of goods. Amounts earned and due from suppliers under these rebate programs are included in other current assets on the Consolidated Balance Sheets and are reflected as a reduction of cost of goods sold on the Consolidated Statements of Operations . Vendor allowances earned reduce the carrying cost of inventory and are recognized in cost of goods sold when the related inventory is sold. CastleGate The Company earns fees from providing logistic services to its suppliers including order fulfillment, warehousing, and inbound supply chain services such as ocean freight and drayage. Fulfillment fees are earned upon completion of preparing customer orders for shipment, warehousing fees are earned upon completion of each storage date, and inbound supply chain services are earned on a straight line basis as the shipments move from origin to destination. CastleGate fees are reflected as a reduction of cost of goods sold . Advertising Costs Advertising consists of direct response performance marketing costs, such as display advertising, paid search advertising, social media advertising, search engine optimization, comparison shopping engine advertising, television advertising, direct mail, catalog and print advertising. Expenditures for advertising are expensed in the period that the advertising first takes place. Advertising expense amounted to $1.1 billion , $774.2 million , and $550.0 million in the years ended December 31, 2019 , 2018 , and 2017 , respectively. Included in prepaid expenses at December 31, 2019 and 2018 are approximately $3.8 million and $0.7 million , respectively, of prepaid advertising costs. Merchant Processing Fees Merchant processing fees totaling $179.7 million , $133.4 million , and $88.7 million in the years ended December 31, 2019 , 2018 , and 2017 , respectively, are included in customer service and merchant fees expense in the Consolidated Statements of Operations . These fees are charged by third parties that provide merchant processing services for customer payments made by credit cards and debit cards. Equity-Based Compensation The Company accounts for its equity-based compensation awards in accordance with ASC Topic 718, Compensation—Stock Compensation (ASC 718). ASC 718 requires all equity-based payments to employees to be recognized as expense in the statements of operations based on their grant date fair values. The Company has granted stock options, restricted shares and restricted stock units. The Company has primarily granted restricted stock units, and to a lesser extent, restricted stock. The Company granted only restricted stock units to employees in the years ended December 31, 2019 , 2018 , and 2017 . Restricted stock values are determined based on the quoted market price of our Class A common stock on the date of grant. The Company accounts for equity awards to non-employees in accordance with FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees, which requires the fair value of an award to non-employees be remeasured at fair value as the award vests. The Company adopted ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting (ASU 2018-07) on January 1, 2019. This ASU simplifies aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. The adoption of ASU 2018-07 did not have a material impact the Company’s Consolidated Balance Sheets, Statements of Operations, Comprehensive Loss, Stockholders' Equity (Deficit) or Cash Flows. The Company adopted ASU 2016-09 "Compensation - Stock Compensation" as of January 1, 2017 using a modified retrospective approach with the option to recognize gross stock compensation expense with actual forfeitures recognized as they occur, with a cumulative-effect adjustment to retained earnings recognized as of January 1, 2017 of $8.7 million . The adoption of ASU 2016-09 also requires all income tax adjustments to be recorded in the Consolidated Statements of Operations . Income Taxes Income taxes are accounted for under the asset and liability method. Under the asset and liability method, 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 their 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 rates is recognized in income in the period that includes the enactment date. The Company records valuation allowances to reduce deferred income tax assets to the amount that is more likely than not to be realized. At December 31, 2019 , we maintain a full valuation allowance against our net worldwide deferred tax asset. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, no amount of benefit attributable to the position is recognized. The tax benefit to be recognized of any tax position that meets the more likely than not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. We evaluate at the end of each reporting period whether some or all of the undistributed earnings of our foreign subsidiaries are permanently reinvested. Our position is based upon several factors including management's evaluation of the Company and its subsidiaries' financial requirements, the short- and long-term operational and fiscal objectives of the Company, and the tax consequences associated with the repatriation of earnings. Hosting Arrangements In August 2018, the FASB issued guidance related to a customer’s accounting for implementation costs incurred in hosting arrangements. The guidance aligns the requirements for capitalizing implementation costs incurred in cloud computing arrangements with the requirements for capitalizing costs to develop or obtain internal-use software. This guidance is effective for interim and annual reporting periods beginning after December 15, 2019, and early adoption is permitted. This guidance may be applied either retrospectively or prospectively. The Company early adopted the guidance on a prospective basis as of January 1, 2019. The adoption of the guidance did not have a material impact on the Company's Consolidated Balance Sheets, Statements of Operations, Comprehensive Loss, Stockholders' Equity (Deficit) or Cash Flows. Net Loss Per Share The Company follows the two-class method when computing net loss per share for its two issued classes of common stock - Class A and Class B. Basic net loss per share is computed by dividing the net loss by the weighted average number of common stock outstanding for the period. For periods in which the Company has reported net losses, diluted net loss per share is the same as basic net loss per share, since dilutive common stock are not assumed to have been issued if their effect is anti-dilutive. Subsequent Events The Company considers events or transactions that have occurred after the balance sheet date of December 31, 2019 , but prior to the filing of the financial statements with the U.S. Securities and Exchange Commission, to provide additional evidence relative to certain estimates or to identify matters that require additional recognition or disclosure. Subsequent events have been evaluated through the filing of these financial statements. Recent Accounting Pronouncements Credit Impairment In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments (ASU 2016-13). This ASU revises how entities account for credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. The Company will adopt the new standard as of January 1, 2020 on a prospective basis. We do not believe the adoption of ASU 2016-13 will have a material impact on the Company's Consolidated Balance Sheets , Statements of Operations, Comprehensive Loss, Stockholders' Equity (Deficit) or Cash Flows.

Marketable Securities and Fair

Marketable Securities and Fair Value Measurements	12 Months Ended
Marketable Securities and Fair Value Measurements	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Marketable Securities and Fair Value Measurements	3. Marketable Securities and Fair Value Measurements Marketable Securities As of December 31, 2019 and 2018 , all of the Company’s marketable securities were classified as available-for-sale and their estimated fair values were $559.9 million and $120.8 million , respectively. The Company periodically reviews its available-for-sale securities for other-than-temporary impairment. The Company considers factors such as the duration, severity and the reason for the decline in value, the potential recovery period, and its intent to sell. As of December 31, 2019 and 2018 , the Company’s available-for-sale securities primarily consisted of corporate bonds and other government obligations that are priced at fair value. During the years ended December 31, 2019 , 2018 , and 2017 , the Company did not recognize any other-than-temporary impairment loss. The maturities of the Company’s long-term marketable securities generally range from one to two years . The cost basis of a marketable security sold is determined by the Company using the specific identification method. During the years ended December 31, 2019 , 2018 , and 2017 , the Company did not have any realized gains or losses. The following tables present details of the Company’s marketable securities as of December 31, 2019 and 2018 : December 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) Short-term: Investment securities $ 404,294 $ 20 $ (62 ) $ 404,252 Long-term: Investment securities 155,616 92 (18 ) 155,690 Total $ 559,910 $ 112 $ (80 ) $ 559,942 December 31, 2018 Amortized Cost Gross Unrealized Losses Estimated Fair Value (in thousands) Short-term: Investment securities $ 114,402 $ (124 ) $ 114,278 Long-term: Investment securities 6,603 (77 ) 6,526 Total $ 121,005 $ (201 ) $ 120,804 Fair Value Measurements The Company's financial assets and liabilities are measured at fair value, which is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date (exit price). The three levels of inputs used to measure fair value are as follows: ▪ Level 1—Unadjusted quoted prices in active markets for identical assets or liabilities ▪ Level 2—Unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or inputs other than quoted prices that are observable or can be corroborated by observable market data for substantially the full-term of the asset or liability ▪ Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the asset or liability This hierarchy requires the Company to use observable market data, when available, and to minimize the use of unobservable inputs when determining fair value. The Company measures its cash equivalents and short- and long-term investments at fair value. The Company classifies its cash equivalents and certificate of deposits within Level 1 because the Company values these investments using quoted market prices. The fair value of the Company's Level 1 financial assets is based on quoted market prices of the identical underlying security. The Company classifies short- and long-term investments within Level 2 because unadjusted quoted prices for identical or similar assets in markets are not active. The Company does not have any assets or liabilities classified as Level 3 financial assets. The following tables set forth the fair value of the Company's financial assets measured at fair value on a recurring basis as of December 31, 2019 and 2018 based on the three-tier value hierarchy described in Note 2 , Summary of Significant Accounting Policies : December 31, 2019 Level 1 Level 2 Level 3 Total (in thousands) Cash and cash equivalents: Cash $ 308,521 $ — $ — $ 308,521 Cash equivalents 274,232 — — 274,232 Total cash and cash equivalents 582,753 — — 582,753 Short-term investments: Investment securities — 404,252 — 404,252 Other non-current assets: Certificate of deposit 5,076 — — 5,076 Long-term: Investment securities — 155,690 — 155,690 Total $ 587,829 $ 559,942 $ — $ 1,147,771 December 31, 2018 Level 1 Level 2 Level 3 Total (in thousands) Cash and cash equivalents: Cash $ 134,375 $ — $ — $ 134,375 Cash equivalents 715,086 — — 715,086 Total cash and cash equivalents 849,461 — — 849,461 Short-term investments: Investment securities — 114,278 — 114,278 Other non-current assets Certificate of deposit 5,000 — — 5,000 Long-term: Investment securities — 6,526 — 6,526 Total $ 854,461 $ 120,804 $ — $ 975,265

Intangible Assets and Goodwill

Intangible Assets and Goodwill	12 Months Ended
Intangible Assets and Goodwill	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets and Goodwill	4. Intangible Assets and Goodwill As of December 31, 2019 and 2018 , the Company had $18.4 million and $0.5 million of intangible assets, respectively. Amortization expense related to intangible assets was $0.8 million , $0.9 million , and $1.1 million for the years ended December 31, 2019 , 2018 , and 2017 , respectively. Goodwill was $0.4 million and $2.1 million for the years ended December 31, 2019 and 2018 , respectively.

Property and Equipment, net

Property and Equipment, net	12 Months Ended
Property and Equipment, net	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Property and Equipment, net	5. Property and Equipment, net The following table summarizes property and equipment, net as of December 31, 2019 and 2018 : December 31, 2019 2018 (in thousands) Furniture and computer equipment $ 509,120 $ 339,761 Site and software development costs 297,252 172,653 Leasehold improvements 228,514 109,929 Construction in progress 45,503 90,104 Buildings (leased - Note 6) — 184,694 1,080,389 897,141 Less accumulated depreciation and amortization (455,845 ) (290,164 ) Property and equipment, net $ 624,544 $ 606,977 Property and equipment depreciation and amortization expense was $191.6 million , $122.6 million , and $85.9 million for the years ended December 31, 2019 , 2018 and 2017 , respectively.

Leases

Leases	12 Months Ended
Leases	Dec. 31, 2019
Leases [Abstract]
Leases	6. Leases After Adoption of ASU 2016-02 The Company has lease arrangements for warehouse, fulfillment center, office, and data center spaces. These leases expire at various dates through 2035 . Operating lease expense was $122.0 million , $66.7 million and $48.4 million in the twelve months ended December 31, 2019 , 2018 and 2017 , respectively. The following table presents other information related to leases: Year Ended December 31, 2019 (in thousands) Supplemental cash flows information: Cash payments included in operating cash flows from lease arrangements $ 109,163 Right-of-use assets obtained in exchange for lease obligations $ 301,053 December 31, 2019 Additional lease information: Weighted average remaining lease term 10 years Weighted average discount rate 6.7 % Future minimum lease payments under non-cancellable leases as of December 31, 2019 were as follows: Amount (in thousands) 2020 $ 143,398 2021 150,025 2022 141,866 2023 137,276 2024 134,724 Thereafter 526,671 Total future minimum lease payments 1,233,960 Less: Imputed interest (320,254 ) Total $ 913,706 The following table presents total operating leases: December 31, 2019 (in thousands) Balance sheet line item: Other current liabilities 91,104 Operating lease liabilities 822,602 Total operating leases $ 913,706 As of December 31, 2019 , the Company has entered into $342.3 million of additional operating leases, primarily related to build-to-suit warehouse leases that have not yet commenced. As the Company does not control the underlying assets during the construction period, the Company is not considered the owner of the construction projects for accounting purposes. These operating leases will commence between 2020 and 2021 with lease terms of 5 to 15 years . Before Adoption of ASU 2016-02 The Company established assets and liabilities for the estimated construction costs incurred under lease arrangements where the Company is considered the owner for accounting purposes only to the extent the Company is involved in the construction of structural improvements or takes construction risk prior to commencement of a lease. These are referred to as "build-to-suit leases". Upon occupancy of facilities under build-to-suit leases, the Company assesses whether these arrangements qualify for sales recognition under the sale-leaseback accounting guidance. If the Company continues to be the deemed owner, the facilities are accounted for as financing leases. In the year ended December 31, 2018, the construction efforts related to three warehouse lease arrangements were completed. In the first warehouse lease arrangement, the Company concluded it had a letter of credit of $2.5 million and as a result the Company did not meet the sale-leaseback criteria for derecognition of the building assets and liabilities. In both the second and third warehouse lease arrangements, the Company provided non-recourse financing to the lessor and as a result the Company did not meet the sale-leaseback criteria for derecognition of the building assets and liabilities. Accordingly, these leases were accounted for as financing leases and $101.0 million was recorded in lease financing obligations, net of current portion and property and equipment, net , respectively, in the Company’s Consolidated Balance Sheets as of December 31, 2018.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities	12 Months Ended
	Dec. 31, 2019
Payables and Accruals [Abstract]
Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities	7. Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities The following table presents the components of selected balance sheet items as of December 31, 2019 and 2018 : December 31, 2019 2018 (in thousands) Prepaid expenses and other current assets: Deferred costs in transit $ 104,947 $ 83,652 Prepaid assets 46,177 31,003 Supplier receivable 28,245 31,842 Supplier credits receivable 20,083 17,667 Other current assets 29,269 31,266 Total prepaid expenses and other current assets $ 228,721 $ 195,430 December 31, 2019 2018 (in thousands) Accrued expenses: Employee compensation and related benefits $ 141,922 $ 91,914 Advertising 71,597 67,259 Credit card 13,943 16,740 Accrued property, plant and equipment 18,348 13,955 Audit, legal and professional fees 4,018 1,703 Other accrued expenses 49,090 21,426 Total accrued expenses $ 298,918 $ 212,997 December 31, 2019 2018 (in thousands) Other current liabilities: Short term lease liability (1) $ 91,104 $ — Sales tax payable 62,173 49,989 Sales return reserve 38,042 35,707 Other current liabilities 45,544 42,299 Total current other liabilities $ 236,863 $ 127,995 December 31, 2019 2018 (in thousands) Other liabilities: Construction costs under build-to-suit leases (1) $ — $ 60,089 Deferred rent (1) — 99,746 Other liabilities 6,940 553 Total other liabilities $ 6,940 $ 160,388 (1) On January 1, 2019 the Company adopted ASU 2016-02. Refer to Note 2 , Summary of Significant Accounting Policies

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. Commitments and Contingencies Letters of Credit The Company has issued letters of credit, primarily as security for certain lease agreements, for approximately $46.7 million and $26.8 million as of December 31, 2019 and 2018 , respectively. Purchase Obligations The Company has entered into purchase obligations which represent enforceable and legally binding software license commitments. Contracts that are able to be canceled, both in full or in part, are not included in the table below as they do not represent a legally binding arrangement. The following table summarizes our purchase obligations as of December 31, 2019 : Payments Due During Year Ended December 31, 2020 2021 2022 2023 2024 Thereafter Total (in thousands) Purchase Obligations $ 21,206 $ 25,759 $ 15,653 $ 2,047 $ 1,195 $ — $ 65,860 Collection of Sales or Other Similar Taxes The Company has historically collected and remitted sales tax based on the locations of its physical operations. On June 21, 2018, the U.S. Supreme Court rendered a 5-4 majority decision in South Dakota v. Wayfair Inc., 17-494. Among other things, the Court held that a state may require an out-of-state seller with no physical presence in the state to collect and remit sales taxes on goods the seller ships to consumers in the state, overturning existing court precedent. Several states and other taxing jurisdictions have presented, or indicated that they may present, the Company with sales tax assessments. The aggregate assessments received as of December 31, 2019 are not material to the Company's business and the Company does not expect the Court's decision to have a significant impact on its business. Legal Matters On January 10, 2019 and January 16, 2019, putative securities class action complaints were filed against the Company and three of its officers in the U.S. District Court for the District of Massachusetts. The two complaints allege violations of Sections 10(b) and 20(a) of the Securities Exchange Act of 1934, as amended, relating to certain prior disclosures of the Company. Each plaintiff seeks to represent a class of shareholders who purchased or acquired stock of the Company between August 2, 2018 and October 31, 2018 and seeks damages and other relief based on allegations that the defendants' conduct affected the value of such stock. The Company intends to defend these lawsuits vigorously. On August 30, 2019, the Company filed a motion to dismiss the complaint with prejudice. At this time, based on available information regarding this litigation, the Company is unable to reasonably assess the ultimate outcome of these cases or determine an estimate, or a range of estimates, of potential losses. From time to time the Company is involved in claims that arise during the ordinary course of business. Although the results of litigation and claims cannot be predicted with certainty, the Company does not currently believe that the outcome of any of these other legal matters will have a material adverse effect on the Company's results of operation or financial condition. Regardless of the outcome, litigation can be costly and time consuming, as it can divert management's attention from important business matters and initiatives, negatively impacting the Company's overall operations. In addition, the Company may also find itself at greater risk to outside party claims as it increases its operations in jurisdictions where the laws with respect to the potential liability of online retailers are uncertain, unfavorable, or unclear.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
Employee Benefit Plans	Dec. 31, 2019
Retirement Benefits [Abstract]
Employee Benefit Plans	9. Employee Benefit Plans The Company has a defined-contribution, incentive savings plan pursuant to Section 401(k) of the Internal Revenue Code. The plan covers all full-time employees who have reached the age of 21 years. Employees may elect to defer compensation up to a dollar limit (as allowable by the Internal Revenue Code), of which up to 4% of an employee's salary will be matched by the Company. The amounts deferred by the employee and the matching amounts contributed by the Company both vest immediately. The amount expensed under the plan totaled approximately $27.9 million , $18.2 million , and $9.0 million in the years ended December 31, 2019 , 2018 and 2017 , respectively.

Equity-Based Compensation

Equity-Based Compensation	12 Months Ended
Equity-Based Compensation	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Equity-Based Compensation	10. Equity-Based Compensation The board of directors of the Company (the "Board") adopted the 2014 Incentive Award Plan ("2014 Plan") to grant cash and equity incentive awards to eligible participants in order to attract, motivate and retain talent. The 2014 Plan is administered by the Board with respect to awards to non-employee directors and by the compensation committee of the Board with respect to other participants and provides for the issuance of stock options, SARs, restricted stock, restricted stock units ("RSUs"), performance shares, stock payments, cash payments, dividend awards and other incentives. Prior to the adoption of the 2014 Plan, Wayfair LLC issued certain equity awards pursuant to the Wayfair LLC Amended and Restated Common Unit Plan (the "2010 Plan"), which was administered by the board of directors of Wayfair LLC. Awards issued under the 2010 Plan that remain outstanding currently represent Class A or Class B common stock of the Company. 8,603,066 shares of Class A common stock were initially available for issuance under awards granted pursuant to the 2014 Plan. The 2014 Plan also contains an evergreen provision whereby the shares available for future grant are increased on the first day of each calendar year beginning January 1, 2016 and ending on and including January 1, 2024. As of January 1, 2020 , 5,111,305 shares of Class A common stock were available for future grant under the 2014 Plan. Shares or RSUs forfeited, withheld for minimum statutory tax obligations, and unexercised stock option lapses from the 2010 and 2014 Plans are available for future grant under the 2014 Plan. The following table presents activity relating to stock options for the year ended December 31, 2019 : Shares Weighted-Average Exercise Price Weighted-Average Remaining Contractual Term (Years) Outstanding at December 31, 2018 79,802 $ 3.05 2.5 Options exercised (35,996 ) $ 3.11 Options expired (200 ) $ 3.42 Outstanding and exercisable at December 31, 2019 43,606 $ 3.00 1.5 Intrinsic value of stock options exercised was $4.8 million and $4.3 million for the years ended December 31, 2019 and 2018 , respectively. Aggregate intrinsic value of stock options outstanding and currently exercisable is $3.8 million as of December 31, 2019 . All stock options were fully vested at December 31, 2019 . The following table presents activity relating to restricted common stock for the year ended December 31, 2019 : Shares Weighted-Average Grant Date Fair Value Unvested at December 31, 2018 20,000 $ 44.34 Restricted stock vested (20,000 ) $ 44.34 Unvested as of December 31, 2019 — $ — The intrinsic value of restricted common stock vested was $1.8 million for the years ended December 31, 2019 and 2018 , respectively. The following table presents activity relating to RSUs for the year ended December 31, 2019 : Shares Weighted-Average Grant Date Fair Value Unvested at December 31, 2018 7,970,959 $ 72.43 RSUs granted 4,420,948 $ 120.29 RSUs vested (2,835,165 ) $ 74.47 RSUs forfeited/canceled (1,444,006 ) $ 84.51 Outstanding as of December 31, 2019 8,112,736 $ 95.69 The intrinsic value of RSUs vested was $358.6 million and $262.4 million for the years ended December 31, 2019 and 2018 , respectively. Aggregate intrinsic value of RSUs unvested is $733.1 million as of December 31, 2019 . Unrecognized equity-based compensation expense related to RSUs expected to vest over time is $705.5 million with a weighted average remaining vesting term of 1.4 years as of December 31, 2019 .

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit)	12 Months Ended
Stockholders' Equity (Deficit)	Dec. 31, 2019
Equity [Abstract]
Stockholders' Equity (Deficit)	11. Stockholders’ Equity (Deficit) Preferred Stock The Company authorized 10,000,000 shares of undesignated preferred stock, $0.001 par value per share, for future issuance. As of December 31, 2019 , the Company had no shares of undesignated preferred stock issued or outstanding. Common Stock The Company authorized 500,000,000 shares of Class A common stock, $0.001 par value per share, and 164,000,000 shares of Class B common stock, $0.001 par value per share, of which 66,642,611 and 62,329,701 shares of Class A common stock and 26,957,815 and 28,417,882 shares of Class B common stock were outstanding as of December 31, 2019 and 2018 , respectively. The rights of the holders of Class A common stock and Class B common stock are identical, except with respect to voting and conversion rights. Each share of Class A common stock is entitled to one vote per share and each share of Class B common stock is entitled to ten votes per share. Each share of Class B common stock may be converted into one share of Class A common stock at the option of its holder and will be automatically converted into one share of Class A common stock upon transfer thereof, subject to certain exceptions. In addition, upon the date on which the outstanding shares of Class B common stock represent less than 10% of the aggregate number of shares of the then outstanding Class A common stock and Class B common stock, or in the event of the affirmative vote or written consent of holders of at least 66 2/3% of the outstanding shares of Class B common stock, all outstanding shares of Class B common stock shall convert automatically into Class A common stock. Subject to preferences that may apply to any shares of preferred stock outstanding at the time, the holders of common stock are entitled to receive dividends out of funds legally available if the Board, in its discretion, determines to issue dividends and then only at the times and in the amounts that the Board may determine. Since the Company's initial public offering through December 31, 2019 , 55,081,618 shares of Class B common stock were converted to Class A common stock.

Segment and Geographic Informat

Segment and Geographic Information	12 Months Ended
Segment and Geographic Information	Dec. 31, 2019
Segment Reporting [Abstract]
Segment and Geographic Information	12. Segment and Geographic Information Operating segments are defined as components of an enterprise for which separate financial information is available that is evaluated on a regular basis by the Chief Operating Decision Maker ("CODM") in deciding how to allocate resources to an individual segment and in assessing performance. The Company’s CODM is its Chief Executive Officer. The Company's operating and reportable segments are the U.S. and International. These segments reflect the way the CODM allocates resources and evaluates financial performance, which is based upon each segment's Adjusted EBITDA . Adjusted EBITDA is defined as loss before depreciation and amortization , equity-based compensation and related taxes , interest (expense), net , other (income) expense, net , provision for income taxes, net , non-recurring items , and other items not indicative of our ongoing operating performance . These charges are excluded from evaluation of segment performance because it facilitates reportable segment performance comparisons on a period-to-period basis as these costs may vary independent of business performance. The accounting policies of the segments are the same as those described in Note 2 , Summary of Significant Accounting Policies . The Company allocates certain operating expenses to the operating and reportable segments, including customer service and merchant fees and selling, operations, technology, general and administrative based on the usage and relative contribution provided to the segments. It excludes from the allocations certain operating expense lines, including depreciation and amortization , equity-based compensation and related taxes , interest (expense), net , other (income) expense, net , and provision for income taxes, net . There are no revenue transactions between the Company's reportable segments. U.S. The U.S. segment primarily consists of amounts earned through product sales through the Company's family of sites in the U.S. and through websites operated by third parties in the U.S. The U.S. net revenue for the years ended December 31, 2018 and 2017 include $61.1 million and $77.7 million , respectively, of net revenue previously classified as other net revenue. International The International segment primarily consists of amounts earned through product sales through the Company's international sites. Net revenue from external customers for each group of similar products and services are not reported to the CODM. Separate identification of this information for purposes of segment disclosure is impractical, as it is not readily available and the cost to develop it would be excessive. No individual country outside of the U.S. provided greater than 10% of consolidated net revenue. The following tables present net revenues and Adjusted EBITDA attributable to the Company’s reportable segments for the periods presented: Year Ended December 31, 2019 2018 2017 (in thousands) U.S. net revenue $ 7,764,831 $ 5,813,070 $ 4,153,057 International net revenue 1,362,226 966,104 567,838 Total net revenue $ 9,127,057 $ 6,779,174 $ 4,720,895 Year Ended December 31, 2019 2018 2017 (in thousands) Adjusted EBITDA: U.S. $ (179,010 ) $ (19,049 ) $ 35,888 International (317,534 ) (195,937 ) (102,921 ) Total reportable segments Adjusted EBITDA (496,544 ) (214,986 ) (67,033 ) Less: reconciling items (1) (488,040 ) (289,094 ) (177,581 ) Net loss $ (984,584 ) $ (504,080 ) $ (244,614 ) (1) The following adjustments are made to reconcile total reportable segments Adjusted EBITDA to consolidated net loss: Year Ended December 31, 2019 2018 2017 (in thousands) Depreciation and amortization (1) $ 192,419 $ 123,542 $ 87,020 Equity-based compensation and related taxes 240,978 136,415 72,626 Interest (expense), net 54,514 28,560 9,433 Other (income) expense, net (2,881 ) 204 (758 ) Provision for income taxes, net 3,010 2,037 486 Other (1) — (1,664 ) 8,774 Total reconciling items $ 488,040 $ 289,094 $ 177,581 (1) The Company recorded $9.6 million of one-time charges in the year ended December 31, 2017 in selling, operations, technology, general and administrative in the Consolidated Statements of Operations related to a warehouse the Company vacated in July 2017. Of the $9.6 million charges, $8.8 million was included in other and related primarily to the excess of the Company's estimated future remaining lease commitments through 2023 over its expected sublease income over the same period, and $0.8 million was included in depreciation and amortization related to accelerated depreciation of leasehold improvements in the warehouse. In the year ended December 31, 2018, the Company terminated the lease and recognized in Other a $1.7 million one-time gain related to the difference in the expected future net lease commitments and the actual costs incurred to terminate the lease. The gain was recognized in selling, operations, technology, general and administrative in the Consolidated Statements of Operations . The following table presents long-lived assets attributable to the Company's reportable segments reconciled to the amounts: Year Ended December 31, 2019 2018 (in thousands) Geographic long-lived assets: U.S. $ 731,963 $ 470,260 International 144,803 31,303 Total reportable segment long-lived assets 876,766 501,563 Plus: reconciling corporate long-lived assets 511,178 105,414 Total long-lived assets $ 1,387,944 $ 606,977 U.S. and International long-lived assets consist of property and equipment, net and operating lease right-of-use assets. Corporate long-lived assets consist of property and equipment, net and operating lease right-of-use assets at our corporate facilities. The following table presents total assets attributable to the Company's reportable segments reconciled to consolidated amounts: Year Ended December 31, 2019 2018 (in thousands) Assets by segment: U.S. $ 1,076,733 $ 738,600 International 190,167 55,159 Total reportable segment assets 1,266,900 793,759 Plus: reconciling corporate assets 1,686,148 1,097,091 Total assets $ 2,953,048 $ 1,890,850 U.S. and International segment assets consist primarily of accounts receivable, inventories, prepaid expenses and other current assets, property and equipment, net

Income Taxes

Income Taxes	12 Months Ended
Income Taxes	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	13. Income Taxes The components of the provision for income taxes, net for the years ended December 31, 2019 , 2018 and 2017 is presented below: Year ended December 31, 2019 Current Deferred Total (in thousands) Federal $ — $ — $ — State 874 — 874 Foreign 2,136 — 2,136 $ 3,010 $ — $ 3,010 Year ended December 31, 2018 Current Deferred Total (in thousands) Federal $ — $ (86 ) $ (86 ) State 744 (24 ) 720 Foreign 78 1,325 1,403 $ 822 $ 1,215 $ 2,037 Year ended December 31, 2017 Current Deferred Total (in thousands) Federal $ — $ (31 ) $ (31 ) State 540 8 548 Foreign 942 (973 ) (31 ) $ 1,482 $ (996 ) $ 486 The actual provision for income taxes, net differs from the expected provision for income taxes computed at the U.S. Federal statutory tax rate of 21% due to the following: Year Ended December 31, 2019 2018 2017 (in thousands) Provision for income taxes at the federal statutory rate $ (206,131 ) $ (105,429 ) $ (85,445 ) State income tax expense, net of federal benefit (40,136 ) (22,584 ) (11,432 ) Foreign tax rate differential 24,346 14,976 23,179 Non-deductible equity-based compensation expense 6,604 3,267 1,080 Windfall benefits from equity-based compensation (28,915 ) (29,003 ) (24,168 ) Change in valuation allowance 236,863 119,370 24,209 Change in tax rate (2,293 ) 197 71,919 Limitation on officer's compensation 6,509 5,283 — Debt integration termination — 9,236 — Other 6,163 6,724 1,144 Provision for income taxes, net $ 3,010 $ 2,037 $ 486 We recorded a provision for income taxes, net of $3.0 million , representing an effective tax rate of almost zero . The effective tax rate differs from the U.S. Federal statutory rate of 21% primarily as a result of the valuation allowance maintained against our worldwide net deferred tax asset. The components of loss before income taxes determined by tax jurisdiction, are as follows: Year Ended December 31, 2019 2018 2017 (in thousands) U.S. $ (699,347 ) $ (327,356 ) $ (143,800 ) Foreign (282,227 ) (174,687 ) (100,328 ) Total $ (981,574 ) $ (502,043 ) $ (244,128 ) The tax effects of temporary differences that give rise to significant portions of deferred tax assets and liabilities for the periods presented are as follows: December 31, 2019 2018 (in thousands) Deferred tax assets: Accounts receivable $ 5,909 $ 2,400 Inventories 871 495 Net operating loss carry-forwards 470,199 262,481 Equity-based compensation expense 14,929 9,733 Intangibles 10,675 12,526 Accrued payroll 21,395 13,900 Accrued expenses and reserves 14,539 12,339 Charitable contributions 872 657 Leases 239,259 90,868 Gross deferred tax assets 778,648 405,399 Less: Valuation allowance (412,898 ) (247,454 ) Net deferred tax assets $ 365,750 $ 157,945 Deferred tax liabilities: Prepaid expenses $ (5,930 ) $ (3,030 ) Capitalized technology (27,354 ) (15,427 ) Property and equipment (14,191 ) (73,189 ) Operating lease right-of-use asset (197,680 ) — Goodwill (40 ) (133 ) Convertible debt (120,447 ) (66,166 ) Other (108 ) — Total deferred tax liabilities (365,750 ) (157,945 ) Net deferred tax assets (liabilities) — — Non-current net deferred tax assets (liabilities) $ — $ — The valuation allowance increased by $165.4 million during 2019 . The increase in the valuation allowance is primarily the result of establishing a valuation allowance primarily against the current year operating losses of our U.S. and Irish entities, partially offset by a decrease in valuation allowance through equity as a result of the deferred tax liability recorded related to the Company’s convertible debt issuance. In determining the need for a valuation allowance, the Company has given consideration to the cumulative book income and loss positions of each of its entities as well as its worldwide cumulative loss position. The Company has assessed, on a jurisdictional basis, the available means of recovering deferred tax assets, including the ability to carry-back net operating losses, the existence of reversing temporary differences, the availability of tax planning strategies and available sources of future taxable income. At December 31, 2019 , we maintain a full valuation allowance against our worldwide net deferred tax assets. As of December 31, 2019 , the Company had federal net operating loss carryforwards available to offset future federal taxable income of $1.4 billion . In addition, the Company had state net operating loss carryforwards available in the amount of $1.3 billion which are available to offset future state taxable income. Of the federal net operating loss carryforwards, $427.4 million begin to expire in the year ending December 31, 2034. The remaining $1.0 billion of federal net operating loss carryforwards do not expire. The state net operating loss carryforwards begin to expire in the year ending December 31, 2022. The Company's ability to utilize these federal and state net operating loss carry-forwards may be limited in the future if the Company experiences an ownership change pursuant to Internal Revenue Code Section 382. An ownership change occurs when the ownership percentages of 5% or greater stockholders change by more than 50% over a three-year period. The Company also had foreign net operating loss carry-forwards available to offset future foreign income of $686.9 million . The Canadian net operating loss of $11.8 million will begin to expire in the year ending December 31, 2038. The remaining foreign net operating loss carryforwards do not expire. As of December 31, 2019 , the Company has not provided for U.S. deferred income taxes on undistributed earnings of its foreign subsidiaries of approximately $7.3 million since these earnings are deemed to be indefinitely reinvested. Upon distribution of those earnings in the form of dividends or otherwise, the Company could be subject to income taxes as well as withholding taxes. The amount of taxes attributable to the undistributed earnings is immaterial. The Company establishes reserves for uncertain tax positions based on management's assessment of exposures associated with tax deductions, permanent tax differences and tax credits. The tax reserves are analyzed periodically and adjustments are made as events occur to warrant adjustment to the reserve. The Company's policy is to recognize interest and penalties related to unrecognized tax benefits as a component of the provision for income taxes, net. Reserves for uncertain tax positions as of December 31, 2019 and 2018 are not material and would not impact the effective tax rate if recognized as a result of the valuation allowance maintained against our net deferred tax assets. The Company's policy is to recognize interest and penalties related to unrecognized tax benefits as a component of income tax expense. Related to the unrecognized tax benefits noted above, the Company did not accrue any penalties and interest during 2019 , 2018 , and 2017 , respectively. The Company's tax jurisdictions include the U.S., the UK, Germany, Ireland, Canada, Hong Kong and the British Virgin Islands. The statute of limitations with respect to the Company's U.S. federal income taxes has expired for years prior to 2016 . The relevant state U.S. statutes vary and years prior to 2015 are generally closed. The statute of limitations with respect to the Company's foreign income taxes varies, but has expired for years prior to 2015. However, preceding years remain open to examination by U.S. federal and state and foreign taxing authorities to the extent of future utilization of net operating losses generated in each preceding year.

Credit Agreement

Credit Agreement	12 Months Ended
Credit Agreement	Dec. 31, 2019
Debt Disclosure [Abstract]
Credit Agreement	14. Credit Agreement On February 21, 2019 (the "Closing Date"), the Company, as guarantor, and Wayfair LLC, a wholly-owned subsidiary of the Company, as borrower (the “Borrower”) entered into an Amended and Restated Credit Agreement (the “Amended Credit Agreement”) with Citibank, in its capacity as administrative agent, swing line lender and letter of credit issuer, and certain other lenders party thereto. The Amended Credit Agreement replaced the Company's existing credit facility with Citibank. The Amended Credit Agreement consists of: • A secured revolving credit facility under which the Borrower may borrow up to $165 million , subject to certain sublimits, with a final maturity date of February 21, 2022 (the “Revolver”). • The Borrower also has the right, subject to certain customary conditions, to increase the Revolver by $50 million . • The Revolver has the following sublimits: ◦ a $100 million letter of credit sublimit; and ◦ a $15 million swing line sublimit. The Borrower’s obligations under the Amended Credit Agreement are guaranteed by the Company and certain of its subsidiaries (together, the “Guarantors”). The obligations of the Borrower and the Guarantors are secured by first-priority liens on substantially all of the assets of the Borrower and the Guarantors, including, with certain exceptions, all of the capital stock of the Company’s domestic subsidiaries and 65% of the capital stock of the Company’s first-tier foreign subsidiaries. The proceeds of the Revolver may be used to finance working capital, to refinance certain existing indebtedness and to provide funds for permitted acquisitions, repurchases of equity interests and other general corporate purposes. Borrowings under the Revolver will bear interest through maturity at a variable rate based upon, at the Borrower’s option, either the Eurodollar rate or the base rate (which is the highest of (x) Citibank’s prime rate, (y) one-half of 1.00% in excess of the federal funds effective rate, and (z) 1.00% in excess of the one-month Eurodollar rate), plus, in each case an applicable margin. As of the Closing Date, the applicable margin for Eurodollar rate loans was 1.75% per annum and the applicable margin for base rate loans was 0.75% per annum. The applicable margin is subject to specified changes depending on the Liquidity (as defined in the Amended Credit Agreement) of the Company. Any amounts outstanding under the Revolver are due at maturity. In addition, subject to the terms and conditions set forth in the Amended Credit Agreement, the Borrower is required to make certain mandatory prepayments prior to maturity. The Amended Credit Agreement contains affirmative and negative covenants customarily applicable to senior secured credit facilities, including covenants that, among other things, will limit or restrict the ability of the Borrower and the Guarantors, subject to negotiated exceptions, to incur additional indebtedness and additional liens on their assets, engage in mergers or acquisitions or dispose of assets, pay dividends or make other distributions, voluntarily prepay other indebtedness, enter into transactions with affiliated persons, make investments, and change the nature of their businesses. The Amended Credit Agreement also contains customary events of default, subject to thresholds and grace periods, including, among others, payment default, covenant default, cross default to other material indebtedness, and judgment default. In addition, the Amended Credit Agreement requires the Company to maintain certain levels of Free Cash Flow (as defined in the Amended Credit Agreement). The Company did not borrow any amounts under the Amended Credit Agreement during the years ended December 31, 2019 and 2018 . 15. Convertible Debt On September 15, 2017, the Company issued $431.25 million aggregate principal amount of 0.375% Convertible Senior Notes due 2022 (the "2017 Notes"), which includes the exercise in full of a $56.25 million over-allotment option, to certain financial institutions as the initial purchasers of the 2017 Notes (the "2017 Initial Purchasers"). On September 11, 2017, in connection with the pricing of the 2017 Notes, the Company entered into privately negotiated capped call transactions (the "2017 Base Capped Call Transactions") with two of the 2017 Initial Purchasers and certain other financial institutions (the "2017 Option Counterparties") and, in connection with the exercise in full of the over-allotment option by the 2017 Initial Purchasers, on September 14, 2017, entered into additional capped call transactions (such additional capped call transactions, the "2017 Additional Capped Call Transactions” and, together with the 2017 Base Capped Call Transactions, the "2017 Capped Call Transactions") with the 2017 Option Counterparties. Collectively, the 2017 Capped Call Transactions covered, initially, the number of shares of the Company’s Class A common stock underlying the 2017 Notes, subject to anti-dilution adjustments substantially similar to those applicable to the 2017 Notes. On November 15, 2018, the Company amended and restated the 2017 Capped Call Transactions (the "Restated 2017 Capped Call Transactions") with each of the 2017 Option Counterparties in order to, among other things, provide that the options underlying the Restated 2017 Capped Call Transactions can, at the Company’s option, remain outstanding until September 1, 2022, which is the maturity date for the 2017 Notes, even if all or a portion of the 2017 Notes are converted, repurchased or redeemed prior to such date. In November 2018, the Company issued $575.0 million aggregate principal amount of 1.125% Convertible Senior Notes due 2024 (the "2018 Notes"), which includes the exercise in full of a $75.0 million option granted to the initial purchasers, to certain financial institutions as the initial purchasers of the 2018 Notes (the "2018 Initial Purchasers"). The issuance of $500.0 million of 2018 Notes closed on November 19, 2018 and the additional $75.0 million of additional 2018 Notes, which were issued pursuant to the exercise of the 2018 Initial Purchasers' option to purchase such additional 2018 Notes, closed on November 29, 2018. On November 14, 2018, in connection with the pricing of the 2018 Notes, the Company entered into privately negotiated capped call transactions (the "2018 Base Capped Call Transactions") with one of the 2018 Initial Purchasers and certain other financial institutions (the "2018 Option Counterparties") and, in connection with the exercise in full of the 2018 Initial Purchasers option to purchase such additional 2018 Notes, on November 27, 2018, entered into additional capped call transactions (such additional capped call transactions, the "2018 Additional Capped Call Transactions" and, together with the 2018 Base Capped Call Transactions, the "2018 Capped Call Transactions") with the 2018 Option Counterparties. Collectively, the 2018 Capped Call Transactions cover, initially, the number of shares of the Company’s Class A common stock underlying the 2018 Notes, subject to anti-dilution adjustments substantially similar to those applicable to the 2018 Notes. On August 19, 2019, the Company issued $ 948.75 million aggregate principal amount of 1.00% Convertible Senior Notes due 2026 (the "2019 Notes" and together with the 2017 Notes and 2018 Notes, the "Notes"), which includes the exercise in full of a $123.75 million option granted to the initial purchasers, to certain financial institutions as the initial purchasers of the 2019 Notes (the "2019 Initial Purchasers"). On August 14, 2019, in connection with the pricing of the 2019 Notes, the Company entered into privately negotiated capped call transactions (the "2019 Base Capped Call Transactions") with certain of the 2019 Initial Purchasers or their affiliates and another financial institution (the "2019 Option Counterparties") and, in connection with the exercise in full of the 2019 Initial Purchasers' option to purchase such additional 2019 Notes, on August 16, 2019, entered into additional capped call transactions (such additional capped call transactions, the "2019 Additional Capped Call Transactions" and, together with the 2019 Base Capped Call Transactions, the "2019 Capped Call Transactions") with the 2019 Option Counterparties. Collectively, the 2019 Capped Call Transactions cover, initially, the number of shares of the Company’s Class A common stock underlying the 2019 Notes, subject to anti-dilution adjustments substantially similar to those applicable to the 2019 Notes. The net proceeds from the sale of the 2017 Notes, 2018 Notes, and 2019 Notes were approximately $420.4 million , $562.0 million , and $935.1 million , respectively, after deducting the initial purchasers’ discounts and the offering expenses payable by the Company. The Company used approximately $44.2 million , $93.4 million and $145.7 million , respectively, of the net proceeds from the 2017 Notes, 2018 Notes, and 2019 Notes to pay the cost of the 2017 Capped Call Transactions, the 2018 Capped Call Transactions, and the 2019 Capped Call Transactions, respectively. The Company intends to use the remainder of the net proceeds from the Notes for working capital and general corporate purposes. The Company may also use a portion of the net proceeds to finance acquisitions, strategic transactions, investments or the repayment, purchase or exchange of indebtedness (including its existing convertible notes). The Notes are general unsecured obligations of the Company. The Notes rank senior in right of payment to any of the Company’s future indebtedness that is expressly subordinated in right of payment to the Notes; rank equal in right of payment to the Company’s existing and future unsecured indebtedness that is not so subordinated; are effectively subordinated in right of payment to any of the Company’s secured indebtedness to the extent of the value of the assets securing such indebtedness; and are structurally subordinated to all existing and future indebtedness and liabilities of the Company’s subsidiaries. In accounting for the issuance of the Notes, the Company separated the Notes into liability and equity components. The carrying amount of the liability component was calculated by measuring the fair value of a similar liability that does not have an associated convertible feature. The carrying amount of the equity component, representing the conversion option, which does not meet the criteria for separate accounting as a derivative as it is indexed to the Company's own stock, was determined by deducting the fair value of the liability component from the par value of the Notes. The difference between the principal amount of the Notes and the liability component represents the debt discount, which is recorded as a direct deduction from the related debt liability in the Consolidated Balance Sheets and amortized to interest expense using the effective interest method over the term of the Notes. The effective interest rate of the 2017 Notes, 2018 Notes, and 2019 Notes is 6.0% , 8.1% , and 6.4% , respectively. The equity component of the 2017 Notes, 2018 Notes, and 2019 Notes of approximately $95.8 million , $181.5 million , and $280.3 million , respectively, is included in additional paid-in capital in the Consolidated Balance Sheets and is not remeasured as long as it continues to meet the conditions for equity classification. The Company allocated transaction costs related to the Notes using the same proportions as the proceeds from the Notes. Transaction costs attributable to the liability component were recorded as a direct deduction from the related debt liability in the Consolidated Balance Sheets and amortized to interest expense over the term of the Notes, and transaction costs attributable to the equity component were netted with the equity component in shareholders’ equity. The following table presents the outstanding principal amount and carrying value of the Notes as of the date presented: December 31, 2019 December 31, 2018 2017 Notes 2018 Notes 2019 Notes 2017 Notes 2018 Notes (in thousands) Principal amounts: Principal $ 431,250 $ 575,000 $ 948,750 $ 431,250 $ 575,000 Unamortized debt discount (59,830 ) (161,275 ) (277,700 ) (79,911 ) (187,435 ) Net carrying amount $ 371,420 $ 413,725 $ 671,050 $ 351,339 $ 387,565 The following table presents total interest expense recognized related to the Notes: December 31, 2019 December 31, 2018 December 31, 2017 2017 Notes 2018 Notes 2019 Notes 2017 Notes 2018 Notes 2017 Notes (in thousands) Contractual interest expense $ 1,617 $ 6,469 $ 3,342 $ 1,617 $ 1,455 $ 1,491 Interest cost related to amortization of the debt discount 20,080 26,161 12,205 18,910 2,207 4,338 Total interest expense $ 21,697 $ 32,630 $ 15,547 $ 20,527 $ 3,662 $ 5,829 The estimated fair value of the 2017 Notes was $469.5 million and $465.0 million as of December 31, 2019 and 2018 , respectively. The estimated fair value of the 2018 Notes was $597.0 million and $580.2 million as of December 31, 2019 and 2018 , respectively. The estimated fair value of the 2019 Notes was $834.0 million as of December 31, 2019 . The estimated fair value of the Notes was determined through consideration of quoted market prices. The fair value is classified as Level 2, as defined in Note 3 , Marketable Securities and Fair Value Measurements . The if-converted value of the 2017, 2018 and 2019 Notes did not exceed their respective principal value as of December 31, 2019 . 2017 Notes The 2017 Notes were issued pursuant to an indenture, dated September 15, 2017 (the "2017 Indenture"), between the Company and U.S. Bank National Association, as trustee. The Company pays interest on the 2017 Notes semiannually in arrears at a rate of 0.375% per annum on March 1 and September 1 of each year. The 2017 Notes are convertible based upon an initial conversion rate of 9.61 shares of the Company’s Class A common stock per $1,000 principal amount of 2017 Notes (equivalent to a conversion price of approximately $104.06 per share of the Company’s Class A common stock). The conversion rate will be subject to adjustment upon the occurrence of certain specified events, including certain distributions and dividends to all or substantially all of the holders of the Company’s Class A common stock, but will not be adjusted for accrued and unpaid interest. The Company will settle any conversions of the 2017 Notes in cash, shares of the Company’s Class A common stock or a combination thereof, with the form of consideration determined at the Company’s election. The 2017 Notes will mature on September 1, 2022, unless earlier purchased, redeemed or converted. Prior to June 1, 2022, holders may convert all or a portion of their 2017 Notes only under the following circumstances: (1) during any calendar quarter (and only during such calendar quarter), if the last reported sale price of the Company’s Class A common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the 5 business day period after any 10 consecutive trading day period (the "2017 Notes measurement period") in which the trading price per $1,000 principal amount of 2017 Notes for each trading day of the 2017 Notes measurement period was less than 98% of the product of the last reported sale price of the Company’s Class A common stock and the conversion rate on each such trading day; (3) with respect to any 2017 Notes called for redemption by the Company, at any time prior to the close of business on the second scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On and after June 1, 2022 until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their 2017 Notes at any time, regardless of the foregoing circumstances. Holders of 2017 Notes who convert their 2017 Notes in connection with a notice of a redemption or a make-whole fundamental change (each as defined in the 2017 Indenture) may be entitled to a premium in the form of an increase in the conversion rate of the 2017 Notes. The 2017 Notes are not convertible during the first quarter of 2020 and none of the 2017 Notes has been converted to date. The Company may not redeem the 2017 Notes prior to September 8, 2020. On or after September 8, 2020, the Company may redeem for cash all or part of the 2017 Notes if the last reported sale price of the Company’s Class A common stock equals or exceeds 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including at least one of the five trading days immediately preceding the date on which the Company provides notice of redemption, during any 30 consecutive trading days ending on, and including the trading day immediately preceding the date on which the Company provides notice of the redemption. The redemption price will be 100% of the principal amount of the 2017 Notes to be redeemed, plus accrued and unpaid interest, if any. Upon the occurrence of a fundamental change (as defined in the 2017 Indenture), holders may require the Company to repurchase all or a portion of their 2017 Notes for cash at a price equal to 100% of the principal amount of the 2017 Notes to be repurchased plus any accrued but unpaid interest to, but excluding, the fundamental change repurchase date. The 2017 Indenture contains customary terms and covenants, including that upon certain events of default occurring and continuing, either the Trustee or the holders of not less than 25% in aggregate principal amount of the 2017 Notes then outstanding may declare the entire principal amount of all the 2017 Notes plus accrued interest, if any, to be immediately due and payable. 2018 Notes The 2018 Notes were issued pursuant to an indenture, dated November 19, 2018 (the "2018 Indenture"), between the Company and U.S. Bank National Association, as trustee. The Company will pay interest on the 2018 Notes semiannually in arrears at a rate of 1.125% per annum on May 1 and November 1 of each year commencing on May 1, 2019. The 2018 Notes are convertible based upon an initial conversion rate of 8.5910 shares of the Company’s Class A common stock per $1,000 principal amount of 2018 Notes (equivalent to a conversion price of approximately $116.40 per share of the Company’s Class A common stock). The conversion rate will be subject to adjustment upon the occurrence of certain specified events, including certain distributions and dividends to all or substantially all of the holders of the Company’s Class A common stock, but will not be adjusted for accrued and unpaid interest. The Company will settle any conversions of the 2018 Notes in cash, shares of the Company’s Class A common stock or a combination thereof, with the form of consideration determined at the Company’s election. The 2018 Notes will mature on November 1, 2024, unless earlier purchased, redeemed or converted. Prior to August 1, 2024, holders may convert all or a portion of their 2018 Notes only under the following circumstances: (1) during any calendar quarter (and only during such calendar quarter), if the last reported sale price of the Company’s Class A common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the five business day period after any ten consecutive trading day period (the "2018 Notes measurement period") in which the trading price per $1,000 principal amount of 2018 Notes for each trading day of the 2018 Notes measurement period was less than 98% of the product of the last reported sale price of the Company’s Class A common stock and the conversion rate on each such trading day; (3) with respect to any 2018 Notes called for redemption by the Company, at any time prior to the close of business on the second scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On and after August 1, 2024 until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their 2018 Notes at any time, regardless of the foregoing circumstances. Holders of 2018 Notes who convert their 2018 Notes in connection with a make-whole fundamental change or a notice of redemption (each as defined in the 2018 Indenture) may be entitled to a premium in the form of an increase in the conversion rate of the 2018 Notes. The 2018 Notes are not convertible during the first quarter of 2020 and none of the 2018 Notes has been converted to date. The Company may not redeem the 2018 Notes prior to May 8, 2022. On or after May 8, 2022, the Company may redeem for cash all or part of the 2018 Notes if the last reported sale price of the Company’s Class A common stock equals or exceeds 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including at least one of the five trading days immediately preceding the date on which the Company provides notice of redemption, during any 30 consecutive trading days ending on, and including the trading day immediately preceding the date on which the Company provides notice of the redemption. The redemption price will be 100% of the principal amount of the 2018 Notes to be redeemed, plus accrued and unpaid interest, if any. Upon the occurrence of a fundamental change (as defined in the 2018 Indenture), holders may require the Company to repurchase all or a portion of their 2018 Notes for cash at a price equal to 100% of the principal amount of the 2018 Notes to be repurchased plus any accrued but unpaid interest to, but excluding, the fundamental change repurchase date. The 2018 Indenture contains customary terms and covenants, including that upon certain events of default occurring and continuing, either the Trustee or the holders of not less than 25% in aggregate principal amount of the 2018 Notes then outstanding may declare the entire principal amount of all the 2018 Notes plus accrued interest, if any, to be immediately due and payable. 2019 Notes The 2019 Notes were issued pursuant to an indenture, dated August 19, 2019 (the "2019 Indenture"), between the Company and U.S. Bank National Association, as trustee. The Company will pay interest on the 2019 Notes semiannually in arrears at a rate of 1.00% per annum on February 15 and August 15 of each year commencing on February 15, 2020. The 2019 Notes are convertible based upon an initial conversion rate of 6.7349 shares of the Company’s Class A common stock per $1,000 principal amount of 2019 Notes (equivalent to a conversion price of approximately $148.48 per share of the Company’s Class A common stock). The conversion rate will be subject to adjustment upon the occurrence of certain specified events, including certain distributions and dividends to all or substantially all of the holders of the Company’s Class A common stock, but will not be adjusted for accrued and unpaid interest. The Company will settle any conversions of the 2019 Notes in cash, shares of the Company’s Class A common stock or a combination thereof, with the form of consideration determined at the Company’s election. The 2019 Notes will mature on August 15, 2026, unless earlier purchased, redeemed or converted. Prior to May 15, 2026, holders may convert all or a portion of their 2019 Notes only under the following circumstances: (1) during any calendar quarter commencing after the calendar quarter ending on December 31, 2019 (and only during such calendar quarter), if the last reported sale price of the Company’s Class A common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the five business day period after any ten consecutive trading day period (the “2019 Notes measurement period”) in which the trading price per $1,000 principal amount of 2019 Notes for each trading day of the 2019 Notes measurement period was less than 98% of the product of the last reported sale price of the Company’s Class A common stock and the conversion rate on each such trading day; (3) with respect to any 2019 Notes called for redemption by the Company, at any time prior to the close of business on the second scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On and after May 15, 2026 until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their 2019 Notes at any time, regardless of the foregoing circumstances. Holders of 2019 Notes who convert their 2019 Notes in connection with a make-whole fundamental change or a notice of redemption (each as defined in the 2019 Indenture) may be entitled to a premium in the form of an increase in the conversion rate of the 2019 Notes. The 2019 Notes are not convertible during the first quarter of 2020 and none of the 2019 Notes has been converted to date. The Company may not redeem the 2019 Notes prior to August 20, 2023. On or after August 20, 2023, the Company may redeem for cash all or part of the 2019 Notes if the last reported sale price of the Company’s Class A common stock equals or exceeds 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including at least one of the five trading days immediately preceding the date on which the Company provides notice of redemption, during any 30 consecutive trading days ending on, and including the trading day immediately preceding the date on which the Company provides notice of the redemption. The redemption price will be 100% of the principal amount of the 2019 Notes to be redeemed, plus accrued and unpaid interest, if any. Upon the occurrence of a fundamental change (as defined in the 2019 Indenture), holders may require the Company to repurchase all or a portion of their 2019 Notes for cash at a price equal to 100% of the principal amount of the 2019 Notes to be repurchased plus any accrued but unpaid interest to, but excluding, the fundamental change repurchase date. The 2019 Indenture contains customary terms and covenants, including that upon certain events of default occurring and continuing, either the Trustee or the holders of not less than 25% in aggregate principal amount of the 2019 Notes then outstanding may declare the entire principal amount of all the 2019 Notes plus accrued interest, if any, to be immediately due and payable. Capped Call Transactions The Restated 2017 Capped Call Transactions, 2018 Capped Call Transactions and 2019 Capped Call Transactions (collectively, the "Capped Call Transactions") are expected generally to reduce the potential dilution and/or offset the cash payments the Company is required to make in excess of the principal amount of the Notes upon conversion of the Notes in the event that the market price per share of the Company’s Class A common stock is greater than the strike price of the Capped Call Transactions (which initially corresponds to the initial conversion price of the Notes and is subject to certain adjustments under the terms of the Capped Call Transactions), with such reduction and/or offset subject to a cap based on the cap price of the Capped Call Transactions. The Restated 2017 Capped Call Transactions have an initial cap price of $154.16 per share of the Company’s Class A common stock, which represents a premium of 100% over the last reported sale price of the Company’s Class A common stock on September 11, 2017, which is the date the 2017 Notes priced, and is subject to certain adjustments under the terms of the Restated 2017 Capped Call Transactions. The 2018 Capped Call Transactions have an initial cap price of $219.63 per share of the Company’s Class A common stock, which represents a premium of 150% over the last reported sale price of the Company’s Class A common stock on November 14, 2018, which is the day the 2018 Notes priced, and is subject to certain adjustments under the terms of the 2018 Capped Call Transactions. The 2019 Capped Call Transactions have an initial cap price of $280.15 per share of the Company's Class A common stock, which represents a premium of 150% over the last reported sale price of the Company's Class A common stock on August 14, 2019, which is the day the 2019 Notes priced, and is subject to certain adjustments under the terms of the 2019 Capped Call Transactions. Collectively, the Capped Call Transactions cover, initially, the number of shares of the Company’s Class A common stock underlying the Notes, subject to anti-dilution adjustments substantially similar to those applicable to the Notes. The Capped Call Transactions are separate transactions, in each case, entered into by the Company with the 2017 Option Counterparties, the 2018 Option Counterparties, and 2019 Option Counterparties, and are not part of the terms of the Notes and will not affect any holder’s rights under the Notes. Holders of the Notes will not have any rights with respect to the Capped Call Transactions. The Capped Call Transactions do not meet the criteria for separate accounting as a derivative as they are indexed to the Company's stock. The premiums paid for the Capped Call Transactions have been included as a net reduction to additional paid-in capital within stockholders' equity (deficit).

Convertible Debt

Convertible Debt	12 Months Ended
Convertible Debt	Dec. 31, 2019
Debt Disclosure [Abstract]
Convertible Debt	14. Credit Agreement On February 21, 2019 (the "Closing Date"), the Company, as guarantor, and Wayfair LLC, a wholly-owned subsidiary of the Company, as borrower (the “Borrower”) entered into an Amended and Restated Credit Agreement (the “Amended Credit Agreement”) with Citibank, in its capacity as administrative agent, swing line lender and letter of credit issuer, and certain other lenders party thereto. The Amended Credit Agreement replaced the Company's existing credit facility with Citibank. The Amended Credit Agreement consists of: • A secured revolving credit facility under which the Borrower may borrow up to $165 million , subject to certain sublimits, with a final maturity date of February 21, 2022 (the “Revolver”). • The Borrower also has the right, subject to certain customary conditions, to increase the Revolver by $50 million . • The Revolver has the following sublimits: ◦ a $100 million letter of credit sublimit; and ◦ a $15 million swing line sublimit. The Borrower’s obligations under the Amended Credit Agreement are guaranteed by the Company and certain of its subsidiaries (together, the “Guarantors”). The obligations of the Borrower and the Guarantors are secured by first-priority liens on substantially all of the assets of the Borrower and the Guarantors, including, with certain exceptions, all of the capital stock of the Company’s domestic subsidiaries and 65% of the capital stock of the Company’s first-tier foreign subsidiaries. The proceeds of the Revolver may be used to finance working capital, to refinance certain existing indebtedness and to provide funds for permitted acquisitions, repurchases of equity interests and other general corporate purposes. Borrowings under the Revolver will bear interest through maturity at a variable rate based upon, at the Borrower’s option, either the Eurodollar rate or the base rate (which is the highest of (x) Citibank’s prime rate, (y) one-half of 1.00% in excess of the federal funds effective rate, and (z) 1.00% in excess of the one-month Eurodollar rate), plus, in each case an applicable margin. As of the Closing Date, the applicable margin for Eurodollar rate loans was 1.75% per annum and the applicable margin for base rate loans was 0.75% per annum. The applicable margin is subject to specified changes depending on the Liquidity (as defined in the Amended Credit Agreement) of the Company. Any amounts outstanding under the Revolver are due at maturity. In addition, subject to the terms and conditions set forth in the Amended Credit Agreement, the Borrower is required to make certain mandatory prepayments prior to maturity. The Amended Credit Agreement contains affirmative and negative covenants customarily applicable to senior secured credit facilities, including covenants that, among other things, will limit or restrict the ability of the Borrower and the Guarantors, subject to negotiated exceptions, to incur additional indebtedness and additional liens on their assets, engage in mergers or acquisitions or dispose of assets, pay dividends or make other distributions, voluntarily prepay other indebtedness, enter into transactions with affiliated persons, make investments, and change the nature of their businesses. The Amended Credit Agreement also contains customary events of default, subject to thresholds and grace periods, including, among others, payment default, covenant default, cross default to other material indebtedness, and judgment default. In addition, the Amended Credit Agreement requires the Company to maintain certain levels of Free Cash Flow (as defined in the Amended Credit Agreement). The Company did not borrow any amounts under the Amended Credit Agreement during the years ended December 31, 2019 and 2018 . 15. Convertible Debt On September 15, 2017, the Company issued $431.25 million aggregate principal amount of 0.375% Convertible Senior Notes due 2022 (the "2017 Notes"), which includes the exercise in full of a $56.25 million over-allotment option, to certain financial institutions as the initial purchasers of the 2017 Notes (the "2017 Initial Purchasers"). On September 11, 2017, in connection with the pricing of the 2017 Notes, the Company entered into privately negotiated capped call transactions (the "2017 Base Capped Call Transactions") with two of the 2017 Initial Purchasers and certain other financial institutions (the "2017 Option Counterparties") and, in connection with the exercise in full of the over-allotment option by the 2017 Initial Purchasers, on September 14, 2017, entered into additional capped call transactions (such additional capped call transactions, the "2017 Additional Capped Call Transactions” and, together with the 2017 Base Capped Call Transactions, the "2017 Capped Call Transactions") with the 2017 Option Counterparties. Collectively, the 2017 Capped Call Transactions covered, initially, the number of shares of the Company’s Class A common stock underlying the 2017 Notes, subject to anti-dilution adjustments substantially similar to those applicable to the 2017 Notes. On November 15, 2018, the Company amended and restated the 2017 Capped Call Transactions (the "Restated 2017 Capped Call Transactions") with each of the 2017 Option Counterparties in order to, among other things, provide that the options underlying the Restated 2017 Capped Call Transactions can, at the Company’s option, remain outstanding until September 1, 2022, which is the maturity date for the 2017 Notes, even if all or a portion of the 2017 Notes are converted, repurchased or redeemed prior to such date. In November 2018, the Company issued $575.0 million aggregate principal amount of 1.125% Convertible Senior Notes due 2024 (the "2018 Notes"), which includes the exercise in full of a $75.0 million option granted to the initial purchasers, to certain financial institutions as the initial purchasers of the 2018 Notes (the "2018 Initial Purchasers"). The issuance of $500.0 million of 2018 Notes closed on November 19, 2018 and the additional $75.0 million of additional 2018 Notes, which were issued pursuant to the exercise of the 2018 Initial Purchasers' option to purchase such additional 2018 Notes, closed on November 29, 2018. On November 14, 2018, in connection with the pricing of the 2018 Notes, the Company entered into privately negotiated capped call transactions (the "2018 Base Capped Call Transactions") with one of the 2018 Initial Purchasers and certain other financial institutions (the "2018 Option Counterparties") and, in connection with the exercise in full of the 2018 Initial Purchasers option to purchase such additional 2018 Notes, on November 27, 2018, entered into additional capped call transactions (such additional capped call transactions, the "2018 Additional Capped Call Transactions" and, together with the 2018 Base Capped Call Transactions, the "2018 Capped Call Transactions") with the 2018 Option Counterparties. Collectively, the 2018 Capped Call Transactions cover, initially, the number of shares of the Company’s Class A common stock underlying the 2018 Notes, subject to anti-dilution adjustments substantially similar to those applicable to the 2018 Notes. On August 19, 2019, the Company issued $ 948.75 million aggregate principal amount of 1.00% Convertible Senior Notes due 2026 (the "2019 Notes" and together with the 2017 Notes and 2018 Notes, the "Notes"), which includes the exercise in full of a $123.75 million option granted to the initial purchasers, to certain financial institutions as the initial purchasers of the 2019 Notes (the "2019 Initial Purchasers"). On August 14, 2019, in connection with the pricing of the 2019 Notes, the Company entered into privately negotiated capped call transactions (the "2019 Base Capped Call Transactions") with certain of the 2019 Initial Purchasers or their affiliates and another financial institution (the "2019 Option Counterparties") and, in connection with the exercise in full of the 2019 Initial Purchasers' option to purchase such additional 2019 Notes, on August 16, 2019, entered into additional capped call transactions (such additional capped call transactions, the "2019 Additional Capped Call Transactions" and, together with the 2019 Base Capped Call Transactions, the "2019 Capped Call Transactions") with the 2019 Option Counterparties. Collectively, the 2019 Capped Call Transactions cover, initially, the number of shares of the Company’s Class A common stock underlying the 2019 Notes, subject to anti-dilution adjustments substantially similar to those applicable to the 2019 Notes. The net proceeds from the sale of the 2017 Notes, 2018 Notes, and 2019 Notes were approximately $420.4 million , $562.0 million , and $935.1 million , respectively, after deducting the initial purchasers’ discounts and the offering expenses payable by the Company. The Company used approximately $44.2 million , $93.4 million and $145.7 million , respectively, of the net proceeds from the 2017 Notes, 2018 Notes, and 2019 Notes to pay the cost of the 2017 Capped Call Transactions, the 2018 Capped Call Transactions, and the 2019 Capped Call Transactions, respectively. The Company intends to use the remainder of the net proceeds from the Notes for working capital and general corporate purposes. The Company may also use a portion of the net proceeds to finance acquisitions, strategic transactions, investments or the repayment, purchase or exchange of indebtedness (including its existing convertible notes). The Notes are general unsecured obligations of the Company. The Notes rank senior in right of payment to any of the Company’s future indebtedness that is expressly subordinated in right of payment to the Notes; rank equal in right of payment to the Company’s existing and future unsecured indebtedness that is not so subordinated; are effectively subordinated in right of payment to any of the Company’s secured indebtedness to the extent of the value of the assets securing such indebtedness; and are structurally subordinated to all existing and future indebtedness and liabilities of the Company’s subsidiaries. In accounting for the issuance of the Notes, the Company separated the Notes into liability and equity components. The carrying amount of the liability component was calculated by measuring the fair value of a similar liability that does not have an associated convertible feature. The carrying amount of the equity component, representing the conversion option, which does not meet the criteria for separate accounting as a derivative as it is indexed to the Company's own stock, was determined by deducting the fair value of the liability component from the par value of the Notes. The difference between the principal amount of the Notes and the liability component represents the debt discount, which is recorded as a direct deduction from the related debt liability in the Consolidated Balance Sheets and amortized to interest expense using the effective interest method over the term of the Notes. The effective interest rate of the 2017 Notes, 2018 Notes, and 2019 Notes is 6.0% , 8.1% , and 6.4% , respectively. The equity component of the 2017 Notes, 2018 Notes, and 2019 Notes of approximately $95.8 million , $181.5 million , and $280.3 million , respectively, is included in additional paid-in capital in the Consolidated Balance Sheets and is not remeasured as long as it continues to meet the conditions for equity classification. The Company allocated transaction costs related to the Notes using the same proportions as the proceeds from the Notes. Transaction costs attributable to the liability component were recorded as a direct deduction from the related debt liability in the Consolidated Balance Sheets and amortized to interest expense over the term of the Notes, and transaction costs attributable to the equity component were netted with the equity component in shareholders’ equity. The following table presents the outstanding principal amount and carrying value of the Notes as of the date presented: December 31, 2019 December 31, 2018 2017 Notes 2018 Notes 2019 Notes 2017 Notes 2018 Notes (in thousands) Principal amounts: Principal $ 431,250 $ 575,000 $ 948,750 $ 431,250 $ 575,000 Unamortized debt discount (59,830 ) (161,275 ) (277,700 ) (79,911 ) (187,435 ) Net carrying amount $ 371,420 $ 413,725 $ 671,050 $ 351,339 $ 387,565 The following table presents total interest expense recognized related to the Notes: December 31, 2019 December 31, 2018 December 31, 2017 2017 Notes 2018 Notes 2019 Notes 2017 Notes 2018 Notes 2017 Notes (in thousands) Contractual interest expense $ 1,617 $ 6,469 $ 3,342 $ 1,617 $ 1,455 $ 1,491 Interest cost related to amortization of the debt discount 20,080 26,161 12,205 18,910 2,207 4,338 Total interest expense $ 21,697 $ 32,630 $ 15,547 $ 20,527 $ 3,662 $ 5,829 The estimated fair value of the 2017 Notes was $469.5 million and $465.0 million as of December 31, 2019 and 2018 , respectively. The estimated fair value of the 2018 Notes was $597.0 million and $580.2 million as of December 31, 2019 and 2018 , respectively. The estimated fair value of the 2019 Notes was $834.0 million as of December 31, 2019 . The estimated fair value of the Notes was determined through consideration of quoted market prices. The fair value is classified as Level 2, as defined in Note 3 , Marketable Securities and Fair Value Measurements . The if-converted value of the 2017, 2018 and 2019 Notes did not exceed their respective principal value as of December 31, 2019 . 2017 Notes The 2017 Notes were issued pursuant to an indenture, dated September 15, 2017 (the "2017 Indenture"), between the Company and U.S. Bank National Association, as trustee. The Company pays interest on the 2017 Notes semiannually in arrears at a rate of 0.375% per annum on March 1 and September 1 of each year. The 2017 Notes are convertible based upon an initial conversion rate of 9.61 shares of the Company’s Class A common stock per $1,000 principal amount of 2017 Notes (equivalent to a conversion price of approximately $104.06 per share of the Company’s Class A common stock). The conversion rate will be subject to adjustment upon the occurrence of certain specified events, including certain distributions and dividends to all or substantially all of the holders of the Company’s Class A common stock, but will not be adjusted for accrued and unpaid interest. The Company will settle any conversions of the 2017 Notes in cash, shares of the Company’s Class A common stock or a combination thereof, with the form of consideration determined at the Company’s election. The 2017 Notes will mature on September 1, 2022, unless earlier purchased, redeemed or converted. Prior to June 1, 2022, holders may convert all or a portion of their 2017 Notes only under the following circumstances: (1) during any calendar quarter (and only during such calendar quarter), if the last reported sale price of the Company’s Class A common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the 5 business day period after any 10 consecutive trading day period (the "2017 Notes measurement period") in which the trading price per $1,000 principal amount of 2017 Notes for each trading day of the 2017 Notes measurement period was less than 98% of the product of the last reported sale price of the Company’s Class A common stock and the conversion rate on each such trading day; (3) with respect to any 2017 Notes called for redemption by the Company, at any time prior to the close of business on the second scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On and after June 1, 2022 until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their 2017 Notes at any time, regardless of the foregoing circumstances. Holders of 2017 Notes who convert their 2017 Notes in connection with a notice of a redemption or a make-whole fundamental change (each as defined in the 2017 Indenture) may be entitled to a premium in the form of an increase in the conversion rate of the 2017 Notes. The 2017 Notes are not convertible during the first quarter of 2020 and none of the 2017 Notes has been converted to date. The Company may not redeem the 2017 Notes prior to September 8, 2020. On or after September 8, 2020, the Company may redeem for cash all or part of the 2017 Notes if the last reported sale price of the Company’s Class A common stock equals or exceeds 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including at least one of the five trading days immediately preceding the date on which the Company provides notice of redemption, during any 30 consecutive trading days ending on, and including the trading day immediately preceding the date on which the Company provides notice of the redemption. The redemption price will be 100% of the principal amount of the 2017 Notes to be redeemed, plus accrued and unpaid interest, if any. Upon the occurrence of a fundamental change (as defined in the 2017 Indenture), holders may require the Company to repurchase all or a portion of their 2017 Notes for cash at a price equal to 100% of the principal amount of the 2017 Notes to be repurchased plus any accrued but unpaid interest to, but excluding, the fundamental change repurchase date. The 2017 Indenture contains customary terms and covenants, including that upon certain events of default occurring and continuing, either the Trustee or the holders of not less than 25% in aggregate principal amount of the 2017 Notes then outstanding may declare the entire principal amount of all the 2017 Notes plus accrued interest, if any, to be immediately due and payable. 2018 Notes The 2018 Notes were issued pursuant to an indenture, dated November 19, 2018 (the "2018 Indenture"), between the Company and U.S. Bank National Association, as trustee. The Company will pay interest on the 2018 Notes semiannually in arrears at a rate of 1.125% per annum on May 1 and November 1 of each year commencing on May 1, 2019. The 2018 Notes are convertible based upon an initial conversion rate of 8.5910 shares of the Company’s Class A common stock per $1,000 principal amount of 2018 Notes (equivalent to a conversion price of approximately $116.40 per share of the Company’s Class A common stock). The conversion rate will be subject to adjustment upon the occurrence of certain specified events, including certain distributions and dividends to all or substantially all of the holders of the Company’s Class A common stock, but will not be adjusted for accrued and unpaid interest. The Company will settle any conversions of the 2018 Notes in cash, shares of the Company’s Class A common stock or a combination thereof, with the form of consideration determined at the Company’s election. The 2018 Notes will mature on November 1, 2024, unless earlier purchased, redeemed or converted. Prior to August 1, 2024, holders may convert all or a portion of their 2018 Notes only under the following circumstances: (1) during any calendar quarter (and only during such calendar quarter), if the last reported sale price of the Company’s Class A common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the five business day period after any ten consecutive trading day period (the "2018 Notes measurement period") in which the trading price per $1,000 principal amount of 2018 Notes for each trading day of the 2018 Notes measurement period was less than 98% of the product of the last reported sale price of the Company’s Class A common stock and the conversion rate on each such trading day; (3) with respect to any 2018 Notes called for redemption by the Company, at any time prior to the close of business on the second scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On and after August 1, 2024 until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their 2018 Notes at any time, regardless of the foregoing circumstances. Holders of 2018 Notes who convert their 2018 Notes in connection with a make-whole fundamental change or a notice of redemption (each as defined in the 2018 Indenture) may be entitled to a premium in the form of an increase in the conversion rate of the 2018 Notes. The 2018 Notes are not convertible during the first quarter of 2020 and none of the 2018 Notes has been converted to date. The Company may not redeem the 2018 Notes prior to May 8, 2022. On or after May 8, 2022, the Company may redeem for cash all or part of the 2018 Notes if the last reported sale price of the Company’s Class A common stock equals or exceeds 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including at least one of the five trading days immediately preceding the date on which the Company provides notice of redemption, during any 30 consecutive trading days ending on, and including the trading day immediately preceding the date on which the Company provides notice of the redemption. The redemption price will be 100% of the principal amount of the 2018 Notes to be redeemed, plus accrued and unpaid interest, if any. Upon the occurrence of a fundamental change (as defined in the 2018 Indenture), holders may require the Company to repurchase all or a portion of their 2018 Notes for cash at a price equal to 100% of the principal amount of the 2018 Notes to be repurchased plus any accrued but unpaid interest to, but excluding, the fundamental change repurchase date. The 2018 Indenture contains customary terms and covenants, including that upon certain events of default occurring and continuing, either the Trustee or the holders of not less than 25% in aggregate principal amount of the 2018 Notes then outstanding may declare the entire principal amount of all the 2018 Notes plus accrued interest, if any, to be immediately due and payable. 2019 Notes The 2019 Notes were issued pursuant to an indenture, dated August 19, 2019 (the "2019 Indenture"), between the Company and U.S. Bank National Association, as trustee. The Company will pay interest on the 2019 Notes semiannually in arrears at a rate of 1.00% per annum on February 15 and August 15 of each year commencing on February 15, 2020. The 2019 Notes are convertible based upon an initial conversion rate of 6.7349 shares of the Company’s Class A common stock per $1,000 principal amount of 2019 Notes (equivalent to a conversion price of approximately $148.48 per share of the Company’s Class A common stock). The conversion rate will be subject to adjustment upon the occurrence of certain specified events, including certain distributions and dividends to all or substantially all of the holders of the Company’s Class A common stock, but will not be adjusted for accrued and unpaid interest. The Company will settle any conversions of the 2019 Notes in cash, shares of the Company’s Class A common stock or a combination thereof, with the form of consideration determined at the Company’s election. The 2019 Notes will mature on August 15, 2026, unless earlier purchased, redeemed or converted. Prior to May 15, 2026, holders may convert all or a portion of their 2019 Notes only under the following circumstances: (1) during any calendar quarter commencing after the calendar quarter ending on December 31, 2019 (and only during such calendar quarter), if the last reported sale price of the Company’s Class A common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the five business day period after any ten consecutive trading day period (the “2019 Notes measurement period”) in which the trading price per $1,000 principal amount of 2019 Notes for each trading day of the 2019 Notes measurement period was less than 98% of the product of the last reported sale price of the Company’s Class A common stock and the conversion rate on each such trading day; (3) with respect to any 2019 Notes called for redemption by the Company, at any time prior to the close of business on the second scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On and after May 15, 2026 until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their 2019 Notes at any time, regardless of the foregoing circumstances. Holders of 2019 Notes who convert their 2019 Notes in connection with a make-whole fundamental change or a notice of redemption (each as defined in the 2019 Indenture) may be entitled to a premium in the form of an increase in the conversion rate of the 2019 Notes. The 2019 Notes are not convertible during the first quarter of 2020 and none of the 2019 Notes has been converted to date. The Company may not redeem the 2019 Notes prior to August 20, 2023. On or after August 20, 2023, the Company may redeem for cash all or part of the 2019 Notes if the last reported sale price of the Company’s Class A common stock equals or exceeds 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including at least one of the five trading days immediately preceding the date on which the Company provides notice of redemption, during any 30 consecutive trading days ending on, and including the trading day immediately preceding the date on which the Company provides notice of the redemption. The redemption price will be 100% of the principal amount of the 2019 Notes to be redeemed, plus accrued and unpaid interest, if any. Upon the occurrence of a fundamental change (as defined in the 2019 Indenture), holders may require the Company to repurchase all or a portion of their 2019 Notes for cash at a price equal to 100% of the principal amount of the 2019 Notes to be repurchased plus any accrued but unpaid interest to, but excluding, the fundamental change repurchase date. The 2019 Indenture contains customary terms and covenants, including that upon certain events of default occurring and continuing, either the Trustee or the holders of not less than 25% in aggregate principal amount of the 2019 Notes then outstanding may declare the entire principal amount of all the 2019 Notes plus accrued interest, if any, to be immediately due and payable. Capped Call Transactions The Restated 2017 Capped Call Transactions, 2018 Capped Call Transactions and 2019 Capped Call Transactions (collectively, the "Capped Call Transactions") are expected generally to reduce the potential dilution and/or offset the cash payments the Company is required to make in excess of the principal amount of the Notes upon conversion of the Notes in the event that the market price per share of the Company’s Class A common stock is greater than the strike price of the Capped Call Transactions (which initially corresponds to the initial conversion price of the Notes and is subject to certain adjustments under the terms of the Capped Call Transactions), with such reduction and/or offset subject to a cap based on the cap price of the Capped Call Transactions. The Restated 2017 Capped Call Transactions have an initial cap price of $154.16 per share of the Company’s Class A common stock, which represents a premium of 100% over the last reported sale price of the Company’s Class A common stock on September 11, 2017, which is the date the 2017 Notes priced, and is subject to certain adjustments under the terms of the Restated 2017 Capped Call Transactions. The 2018 Capped Call Transactions have an initial cap price of $219.63 per share of the Company’s Class A common stock, which represents a premium of 150% over the last reported sale price of the Company’s Class A common stock on November 14, 2018, which is the day the 2018 Notes priced, and is subject to certain adjustments under the terms of the 2018 Capped Call Transactions. The 2019 Capped Call Transactions have an initial cap price of $280.15 per share of the Company's Class A common stock, which represents a premium of 150% over the last reported sale price of the Company's Class A common stock on August 14, 2019, which is the day the 2019 Notes priced, and is subject to certain adjustments under the terms of the 2019 Capped Call Transactions. Collectively, the Capped Call Transactions cover, initially, the number of shares of the Company’s Class A common stock underlying the Notes, subject to anti-dilution adjustments substantially similar to those applicable to the Notes. The Capped Call Transactions are separate transactions, in each case, entered into by the Company with the 2017 Option Counterparties, the 2018 Option Counterparties, and 2019 Option Counterparties, and are not part of the terms of the Notes and will not affect any holder’s rights under the Notes. Holders of the Notes will not have any rights with respect to the Capped Call Transactions. The Capped Call Transactions do not meet the criteria for separate accounting as a derivative as they are indexed to the Company's stock. The premiums paid for the Capped Call Transactions have been included as a net reduction to additional paid-in capital within stockholders' equity (deficit).

Net Loss per Share

Net Loss per Share	12 Months Ended
Net Loss per Share	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Loss per Share	16. Net Loss per Share Basic and diluted net loss per share is presented using the two-class method required for participating securities: Class A and Class B common stock. The rights of the holders of Class A and Class B common stock are identical, except with respect to voting and conversion. For more information on the rights of Class A and Class B common stockholders, see Note 11 , Stockholders’ Equity (Deficit) . Basic net loss per share is computed using the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share is computed using the weighted-average number of shares of common stock and, if dilutive, common stock equivalents outstanding during the period. The Company's common stock equivalents consist of shares issuable upon the release of restricted stock units, and to a lesser extent, the incremental shares of common stock issuable upon the exercise of stock options and unvested restricted stock. The dilutive effect of these common stock equivalents is reflected in diluted earnings per share by application of the treasury stock method. The Company's basic and diluted net loss per share are the same because the Company has generated net loss and common stock equivalents are excluded from diluted net loss per share because they have an antidilutive impact. The Company allocates undistributed earnings between the classes on a one -to- one basis when computing net loss per share. As a result, basic and diluted net loss per Class A and Class B shares are equivalent. The following table presents the calculation of basic and diluted net loss per share: Year Ended December 31, 2019 2018 2017 (in thousands, except per share data) Net loss $ (984,584 ) $ (504,080 ) $ (244,614 ) Weighted average common shares used for basic and diluted net loss per share computation 92,200 89,472 86,983 Net loss per common share: Basic and Diluted $ (10.68 ) $ (5.63 ) $ (2.81 ) Dilutive common stock equivalents, representing potentially dilutive common stock options, restricted stock and restricted stock units, of 8.2 million , 8.1 million , and 7.0 million for 2019 , 2018 , and 2017 , respectively, were excluded from diluted earnings per share calculations for these periods because of their anti-dilutive effect. Furthermore, the shares of Class A common stock that would be issuable if the Company elects to settle the Notes in shares were excluded from the diluted earnings per share calculation (using the if-converted method) for the year ended December 31, 2019 because their effect would have been anti-dilutive. The Company may settle the conversions of the Notes in cash, shares of the Company's Class A common stock or any combination thereof at its election. For the 2017 Notes, the number of shares of the Company's Class A common stock issuable at the conversion price of $104.06 per share is expected to be 4.1 million shares, for the 2018 Notes, the number of shares of the Company's Class A common stock issuable at the conversion price of $116.40 is expected to be 4.9 million shares, and for the 2019 Notes, the number of shares of the Company's Class A common stock issuable at the conversion price of $148.48 is expected to be 6.4 million shares. However, the Capped Call Transactions are expected generally to reduce the potential dilution of the Company's Class A common stock upon any conversion of Notes and/or offset the cash payments the Company is required to make in excess of the principal amount of the Notes. Under the Restated 2017 Capped Call Transactions, the number of shares of Class A common stock issuable at the conversion price of $154.16 is expected to be 2.8 million shares. Under the 2018 Capped Call Transactions, the number of shares of Class A common stock issuable at the conversion price of $ 219.63 is expected to be 2.6 million shares. Under the 2019 Capped Call Transactions, the number of shares of Class A common stock issuable at the conversion price of $280.15 is expected to be 3.4 million shares. For more information on the Notes and the Capped Call Transactions, see Note 15 , Convertible Debt .

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying Consolidated Financial Statements have been prepared in conformity with accounting principles generally accepted in the United States of America ("GAAP"). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification ("ASC") and Accounting Standards Update ("ASU") of the Financial Accounting Standards Board ("FASB").
Principles of Consolidation	Principles of Consolidation The accompanying Consolidated Financial Statements of Wayfair Inc. include its wholly-owned subsidiaries (collectively the "Company" or "Wayfair"). All intercompany accounts and transactions have been eliminated. Below is a summary of the wholly-owned subsidiaries of the Company with operations: Subsidiary Location Wayfair LLC U.S. CastleGate Logistics Inc. U.S. SK Retail, Inc. U.S. Wayfair Maine LLC U.S. Wayfair Transportation LLC U.S. Wayfair Securities Corporation U.S. Wayfair Stores Limited Republic of Ireland Wayfair (UK) Limited United Kingdom Wayfair GmbH Germany CastleGate Logistics Canada Inc. Canada CastleGate Logistics Hong Kong Limited Hong Kong Wayfair (BVI) Ltd. British Virgin Islands
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities, at the date of the Consolidated Financial Statements , and the reported amounts of revenue and expenses during the reported period. Actual results could differ from those estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments purchased with an original maturity (at the date of purchase) of three months or less to be the equivalent of cash for the purpose of Consolidated Balance Sheets and Statements of Cash Flows presentation. Cash equivalents, which consist primarily of money market accounts, are carried at cost, which approximates market value.
Short-Term Investments and Marketable Securities	Short-Term Investments and Marketable Securities Short-term investments consist of certificates of deposits and marketable securities with original maturities of greater than three months and maturing in less than twelve months from the balance sheet date. The Company classifies its marketable securities as "available-for-sale" securities. Available-for-sale securities are classified as short- and long-term investments on the Consolidated Balance Sheets and are carried at fair value. Unrealized gains and losses on available-for-sale securities that are considered temporary are recorded, net of taxes, in the accumulated other comprehensive (loss) caption of the Company’s Consolidated Balance Sheets . Unrealized losses, excluding losses related to the credit rating of the security (credit losses), on available-for-sale securities that are considered other-than-temporary but relate to securities that the Company (i) does not intend to sell and (ii) will not be required to sell below cost are also recorded, net of taxes, in accumulated other comprehensive (loss) . Further, the Company does not believe it will be required to sell such securities below cost. Therefore, the only other-than-temporary losses the Company records in other (income) expense, net in its Consolidated Statements of Operations are related to credit losses. As of December 31, 2019 and 2018 , the Company’s available-for-sale securities consisted of corporate bonds and other government obligations that are priced at fair value. The maturities of the Company’s long-term marketable securities generally range from one to two years . The cost basis of a marketable security sold is determined by the Company using the specific identification method.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company's financial assets and liabilities are measured at fair value, which is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date (exit price). The three levels of inputs used to measure fair value are as follows: • Level 1—Unadjusted quoted prices in active markets for identical assets or liabilities • Level 2—Unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or inputs other than quoted prices that are observable or can be corroborated by observable market data for substantially the full-term of the asset or liability • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the asset or liability This hierarchy requires the Company to use observable market data, when available, and to minimize the use of unobservable inputs when determining fair value. The Company measures its cash equivalents and short- and long-term investments at fair value. The Company classifies its cash equivalents within Level 1 because the fair value of the financial asset is valued using quoted market prices of an identical asset. The Company classifies short- and long-term investments within Level 2 because unadjusted quoted prices for identical or similar assets in markets are not active. The Company does not have any assets or liabilities classified as Level 3 financial assets. Refer to Note 3 , Marketable Securities and Fair Value Measurements , for additional detail.
Concentrations of Credit Risk	Concentrations of Credit Risk Financial instruments that subject the Company to credit risk consist of cash and cash equivalents, short- and long-term investments, and accounts receivable. The risk with respect to cash and cash equivalents is minimized by the Company's policy to maintain these balances with major financial institutions of high-credit quality. At times, cash balances may exceed federally insured limits; however, to date, the Company has not incurred any losses on these investments. As of December 31, 2019 and 2018 , the Company had $48.2 million and $129.5 million , respectively, in banks located outside the U.S. The risk with respect to short- and long-term investments is minimized by the Company's policy of investing in financial instruments issued by highly-rated financial institutions. The risk with respect to accounts receivable is managed by the Company through its policy of monitoring the creditworthiness of its customers to which it grants credit terms in the normal course of business.
Accounts Receivable	Accounts Receivable Accounts receivable are stated net of an allowance for doubtful accounts, which is based on historical losses, existing economic conditions, and other information available at the Consolidated Balance Sheets dates. Uncollectible amounts are written off against the allowance after all collection efforts have been exhausted.
Inventories	Inventories Inventories consisting of finished goods are stated at the lower of cost or net realizable value, determined by the first-in, first-out (FIFO) method, and consist of product for resale. Inventory costs consist of cost of product and inbound shipping and handling costs. Inventory costs also include direct and indirect labor costs, rents and depreciation expenses associated with the Company's fulfillment centers. Inventory valuation requires the Company to make judgments, based on currently available information, about the likely method of disposition, such as through sales to individual customers, liquidations, and expected recoverable values of each disposition category.
Goods In-Transit	Goods In-Transit Goods in-transit to customers are recorded in prepaid expenses and other current assets in the Consolidated Balance Sheets
Property and Equipment	Property and Equipment Property and equipment are stated at cost, net of depreciation and amortization . Expenditures for maintenance and repairs are charged to expense as incurred, whereas betterments are capitalized as additions to property and equipment. Depreciation and amortization on property and equipment is calculated on the straight-line method over the estimated useful lives of the assets as follows: Class Range of Life (In Years) Furniture and computer equipment 3 to 7 Site and software development costs 2 Leasehold improvements The lesser of useful life or lease term Buildings (leased - Note 6) 30
Site and Software Development Costs	Site and Software Development Costs The Company capitalizes certain costs associated with the development of its sites and internal-use software products after the preliminary project stage is complete and until the software is ready for its intended use. The capitalized costs are amortized over a two-year period. Costs incurred in the preliminary stages of development, after the software is ready for its intended use and for maintenance of internal-use software are expensed as incurred. Upgrade and enhancements are capitalized to the extent they will result in added functionality.
Long-Lived Assets	Long-Lived Assets
Leases	Leases The Company adopted ASU No. 2016-02, "Leases" (ASU 2016-02) on January 1, 2019 using the modified retrospective approach. The Company also elected the package of practical expedients, which among other things, allowed the Company to carryforward historical lease classification. The adoption of the standard resulted in (1) the derecognition of building assets and finance lease obligations for certain leases that did not pass the sale-leaseback criteria, (2) the derecognition of construction in progress assets and other long-term liabilities for certain lease arrangements whereby the Company is no longer considered the deemed construction owner of the construction projects, and (3) the recognition of operating lease right-of-use ("ROU") assets and lease liabilities for lease arrangements with an initial term greater than twelve months. Adoption of ASU 2016-02 resulted in the following adjustments to the Consolidated Balance Sheet: December 31, 2018 ASU 2016-02 Adjustments January 1, 2019 (in thousands) Balance sheet line item: Property and equipment, net $ 606,977 $ (227,709 ) $ 379,268 Operating lease right-of-use assets $ — $ 524,082 $ 524,082 Other noncurrent assets $ 18,826 $ (6,814 ) $ 12,012 Other liabilities $ 160,388 $ (155,996 ) $ 4,392 Other current liabilities $ 127,995 $ (9,624 ) $ 118,371 Lease financing obligations, net of current portion $ 183,056 $ (183,056 ) $ — Operating lease liabilities $ — $ 636,385 $ 636,385 Accumulated deficit $ (1,082,689 ) $ 1,850 $ (1,080,839 ) The adoption of the standard did not have a notable impact on the Company's liquidity or a materially adverse impact on the Company's debt covenant compliance. The Company determines if an arrangement is a lease at inception. Operating leases are included in operating lease right-of-use assets, other current liabilities, and operating lease liabilities on our Consolidated Balance Sheets . As of December 31, 2019 , the Company has not entered into material financing lease arrangements. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. The Company's lease terms may include options to extend or terminate the lease when it is reasonably certain that management will exercise that option. The Company has lease arrangements with lease and non-lease components. For the Company's warehouse and fulfillment center lease arrangements, the Company accounts for lease and non-lease components separately. For all other lease arrangements, the Company accounts for lease and non-lease components as a single lease component. Operating lease ROU assets and lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at commencement date. As most of the Company's leases do not provide an implicit rate, the Company uses its estimated incremental borrowing rate ("IBR") based on the information available at commencement date in determining the present value of future payments. The determination of the Company’s IBR requires judgment. The IBR for each lease is primarily based on publicly-available information for companies within the same industry and with similar credit profiles. The rate is then adjusted for the impact of collateralization, the lease term, and other specific terms included in the Company’s lease arrangements. The IBR was determined at lease commencement, or as of January 1, 2019 for operating leases existing upon adoption of ASC 842. The IBR is subsequently reassessed upon a modification to the lease arrangement. The operating lease ROU asset also includes any lease payments made prior to commencement date and excludes lease incentives and initial direct costs incurred. ROU assets are subsequently assessed for impairment in accordance with the Company’s accounting policy for long-lived assets.
Foreign Currency Translation	Foreign Currency Translation The functional currency of the Company is the U.S. dollar, while the functional currency of certain wholly-owned subsidiaries outside of the U.S. is as follows: Subsidiary Currency Wayfair Stores Limited Euro Wayfair GmbH Euro Wayfair (BVI) Ltd. Euro Wayfair (UK) Limited Pound sterling CastleGate Logistics Canada Inc. Canadian dollar CastleGate Logistics Hong Kong Limited Hong Kong dollar The financial statements of the Company are translated to U.S. dollars using year-end exchange rates as to assets and liabilities and average exchange rates as to revenue and expenses. Capital accounts are translated at their historical exchange rates when the capital transaction occurred. The effects of foreign currency translation are included in other comprehensive loss in the Consolidated Statements of Comprehensive Loss . Transaction gains and losses are included in the Company's Consolidated Statements of Operations
Contingent Liabilities	Contingent Liabilities The Company has certain contingent liabilities that arise in the ordinary course of business activities. The Company accrues for loss contingencies when losses become probable and are reasonably estimable. If the reasonable estimate of the loss is a range and no amount within the range is a better estimate, the minimum amount of the range is recorded as a liability. The Company does not accrue for contingent losses that, in its judgment, are considered to be reasonably possible, but not probable; however, it discloses the range of such reasonably possible losses.
Revenue Recognition	Revenue Recognition The Company primarily generated revenue through product sales on its family of sites and through (i) product sales on websites operated by third parties and (ii) fees earned for media solutions. The Company recognizes net revenue on product sales through the Company's family of sites and third party operated websites using the gross method when the Company has concluded it controls the product before it is transferred to the customer. The Company controls products when it is the entity responsible for fulfilling the promise to the customer and takes responsibility for the acceptability of the goods, assumes inventory risk from shipment through the delivery date, has discretion in establishing prices, and selects the suppliers of products sold. The Company recognizes net revenue from sales of its products upon delivery to the customer. As the Company ships a large volume of packages through multiple carriers, actual delivery dates may not always be available and as such the Company estimates delivery dates based on historical data. Net revenue from product sales includes shipping costs charged to the customer and is recorded net of taxes collected from customers, which are remitted to governmental authorities. Cash discounts and rebates earned by customers at the time of purchase are deducted from gross revenue in determining net revenue. Allowances for sales returns are estimated and recorded based on prior returns history, recent trends, and projections for returns on sales in the current period. Allowances for sales returns at December 31, 2019 and 2018 , were $ 38.0 million and $35.7 million , respectively. Actual returns in subsequent periods have been consistent with estimated amounts. The Company also earns revenue through third-party advertisers that pay based on the number of advertisement related clicks, actions, or impressions for advertisements placed on the Company's sites. Revenue earned under these arrangements is included in net revenue and is recognized in the period in which the click, action, or impression occurs. Net revenue from contracts with customers is disaggregated by geographic region because this manner of disaggregation best depicts how the nature, amount, timing, and uncertainty of revenue and cash flows are affected by economic factors. Refer to Note 12 , Segment and Geographic Information , for additional detail. The Company has three types of contractual liabilities: (i) cash collections from its customers prior to delivery of products purchased, which are initially recorded in unearned revenue , and are recognized as net revenue when the products are delivered, (ii) unredeemed gift cards and site credits, which are initially recorded in unearned revenue, and are recognized in the period they are redeemed, and (iii) membership rewards redeemable for future purchases, which are earned by customers on purchases made with the Company's Wayfair branded, private label credit card, and are initially recorded in other current liabilities, and are recognized as net revenue when redeemed. The portion of gift cards and store credits not expected to be redeemed are recognized as net revenue based on historical redemption pattern which is substantially within twenty-four months from the date of issuance. Contractual liabilities included in unearned revenue and other current liabilities in the Consolidated Balance Sheets were $167.6 million and $4.6 million at December 31, 2019 and $148.1 million and $3.1 million at December 31, 2018 , respectively. During the year ended December 31, 2019 , the Company recognized $126.0 million and $3.1 million of net revenue included in unearned revenue and other current liabilities, respectively, which was recorded as of December 31, 2018 in the Consolidated Balance Sheets . The Company adopted ASU 2014-09 as of January 1, 2018 using a modified retrospective approach and recognized a $4.7 million cumulative-effect adjustment to reduced accumulated deficit. The cumulative-effect adjustment to accumulated deficit was due to breakage of gift cards and site credits, to the extent there is no requirement for remitting balances to governmental agencies. Prior period balances were not retrospectively adjusted. The adoption of ASU 2014-09 did not have a material impact on the Company's Consolidated Balance Sheet and financial results for the year ended December 31, 2018 .
Costs of Goods Sold	Cost of Goods Sold Cost of goods sold consist of: Product Costs: The Company recognizes costs related to the purchase price of products sold, expenses capitalized into Wayfair inventory, which include direct and indirect labor costs, rent, and depreciation expenses, and inbound shipping and handling costs for Wayfair inventory. Shipping and Fulfillment Costs: Shipping costs include outbound shipping costs. Fulfillment costs include costs incurred to operate and staff our fulfillment centers and provide other inbound supply chain services such as ocean freight and drayage. Costs to operate and staff the CastleGate and WDN networks include rent and depreciation expenses associated with various facilities, costs to receive, inspect, pick, package and prepare customer orders for delivery, and direct and indirect labor costs including payroll, payroll-related benefits, and equity-based compensation. Vendor Rebates The Company earns rebates on incentive programs with its suppliers. These rebates are earned upon shipment of goods. Amounts earned and due from suppliers under these rebate programs are included in other current assets on the Consolidated Balance Sheets and are reflected as a reduction of cost of goods sold on the Consolidated Statements of Operations . Vendor allowances earned reduce the carrying cost of inventory and are recognized in cost of goods sold when the related inventory is sold. CastleGate The Company earns fees from providing logistic services to its suppliers including order fulfillment, warehousing, and inbound supply chain services such as ocean freight and drayage. Fulfillment fees are earned upon completion of preparing customer orders for shipment, warehousing fees are earned upon completion of each storage date, and inbound supply chain services are earned on a straight line basis as the shipments move from origin to destination. CastleGate fees are reflected as a reduction of cost of goods sold .
Advertising Costs	Advertising Costs Advertising
Merchant Processing Fees	Merchant Processing Fees Merchant processing fees totaling $179.7 million , $133.4 million , and $88.7 million in the years ended December 31, 2019 , 2018 , and 2017 , respectively, are included in customer service and merchant fees expense in the Consolidated Statements of Operations . These fees are charged by third parties that provide merchant processing services for customer payments made by credit cards and debit cards.
Equity-Based Compensation	Equity-Based Compensation The Company accounts for its equity-based compensation awards in accordance with ASC Topic 718, Compensation—Stock Compensation (ASC 718). ASC 718 requires all equity-based payments to employees to be recognized as expense in the statements of operations based on their grant date fair values. The Company has granted stock options, restricted shares and restricted stock units. The Company has primarily granted restricted stock units, and to a lesser extent, restricted stock. The Company granted only restricted stock units to employees in the years ended December 31, 2019 , 2018 , and 2017 . Restricted stock values are determined based on the quoted market price of our Class A common stock on the date of grant. The Company accounts for equity awards to non-employees in accordance with FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees, which requires the fair value of an award to non-employees be remeasured at fair value as the award vests. The Company adopted ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting (ASU 2018-07) on January 1, 2019. This ASU simplifies aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. The adoption of ASU 2018-07 did not have a material impact the Company’s Consolidated Balance Sheets, Statements of Operations, Comprehensive Loss, Stockholders' Equity (Deficit) or Cash Flows. The Company adopted ASU 2016-09 "Compensation - Stock Compensation" as of January 1, 2017 using a modified retrospective approach with the option to recognize gross stock compensation expense with actual forfeitures recognized as they occur, with a cumulative-effect adjustment to retained earnings recognized as of January 1, 2017 of $8.7 million . The adoption of ASU 2016-09 also requires all income tax adjustments to be recorded in the Consolidated Statements of Operations .
Income Taxes	Income Taxes Income taxes are accounted for under the asset and liability method. Under the asset and liability method, 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 their 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 rates is recognized in income in the period that includes the enactment date. The Company records valuation allowances to reduce deferred income tax assets to the amount that is more likely than not to be realized. At December 31, 2019 , we maintain a full valuation allowance against our net worldwide deferred tax asset. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, no amount of benefit attributable to the position is recognized. The tax benefit to be recognized of any tax position that meets the more likely than not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. We evaluate at the end of each reporting period whether some or all of the undistributed earnings of our foreign subsidiaries are permanently reinvested. Our position is based upon several factors including management's evaluation of the Company and its subsidiaries' financial requirements, the short- and long-term operational and fiscal objectives of the Company, and the tax consequences associated with the repatriation of earnings.
Hosting Arrangements	Hosting Arrangements In August 2018, the FASB issued guidance related to a customer’s accounting for implementation costs incurred in hosting arrangements. The guidance aligns the requirements for capitalizing implementation costs incurred in cloud computing arrangements with the requirements for capitalizing costs to develop or obtain internal-use software. This guidance is effective for interim and annual reporting periods beginning after December 15, 2019, and early adoption is permitted. This guidance may be applied either retrospectively or prospectively. The Company early adopted the guidance on a prospective basis as of January 1, 2019. The adoption of the guidance did not have a material impact on the Company's Consolidated Balance Sheets, Statements of Operations, Comprehensive Loss, Stockholders' Equity (Deficit) or Cash Flows.
Net Loss Per Share	Net Loss Per Share The Company follows the two-class method when computing net loss per share for its two issued classes of common stock - Class A and Class B. Basic net loss per share is computed by dividing the net loss by the weighted average number of common stock outstanding for the period. For periods in which the Company has reported net losses, diluted net loss per share is the same as basic net loss per share, since dilutive common stock are not assumed to have been issued if their effect is anti-dilutive.
Subsequent Events	Subsequent Events The Company considers events or transactions that have occurred after the balance sheet date of December 31, 2019
Recent Accounting Pronouncements, Credit Impairment	Recent Accounting Pronouncements Credit Impairment In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments (ASU 2016-13). This ASU revises how entities account for credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. The Company will adopt the new standard as of January 1, 2020 on a prospective basis. We do not believe the adoption of ASU 2016-13 will have a material impact on the Company's Consolidated Balance Sheets , Statements of Operations, Comprehensive Loss, Stockholders' Equity (Deficit) or Cash Flows.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
Summary of Significant Accounting Policies (Tables)	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of the Wholly-Owned Subsidiaries of the Company	Below is a summary of the wholly-owned subsidiaries of the Company with operations: Subsidiary Location Wayfair LLC U.S. CastleGate Logistics Inc. U.S. SK Retail, Inc. U.S. Wayfair Maine LLC U.S. Wayfair Transportation LLC U.S. Wayfair Securities Corporation U.S. Wayfair Stores Limited Republic of Ireland Wayfair (UK) Limited United Kingdom Wayfair GmbH Germany CastleGate Logistics Canada Inc. Canada CastleGate Logistics Hong Kong Limited Hong Kong Wayfair (BVI) Ltd. British Virgin Islands
Schedule of Estimated Useful Lives Property Plant and Equipment	Depreciation and amortization on property and equipment is calculated on the straight-line method over the estimated useful lives of the assets as follows: Class Range of Life (In Years) Furniture and computer equipment 3 to 7 Site and software development costs 2 Leasehold improvements The lesser of useful life or lease term Buildings (leased - Note 6) 30
Schedule of Adoption of ASU 2016-02	Adoption of ASU 2016-02 resulted in the following adjustments to the Consolidated Balance Sheet: December 31, 2018 ASU 2016-02 Adjustments January 1, 2019 (in thousands) Balance sheet line item: Property and equipment, net $ 606,977 $ (227,709 ) $ 379,268 Operating lease right-of-use assets $ — $ 524,082 $ 524,082 Other noncurrent assets $ 18,826 $ (6,814 ) $ 12,012 Other liabilities $ 160,388 $ (155,996 ) $ 4,392 Other current liabilities $ 127,995 $ (9,624 ) $ 118,371 Lease financing obligations, net of current portion $ 183,056 $ (183,056 ) $ — Operating lease liabilities $ — $ 636,385 $ 636,385 Accumulated deficit $ (1,082,689 ) $ 1,850 $ (1,080,839 )
Summary of Subsidiaries' Functional Currencies	The functional currency of the Company is the U.S. dollar, while the functional currency of certain wholly-owned subsidiaries outside of the U.S. is as follows: Subsidiary Currency Wayfair Stores Limited Euro Wayfair GmbH Euro Wayfair (BVI) Ltd. Euro Wayfair (UK) Limited Pound sterling CastleGate Logistics Canada Inc. Canadian dollar CastleGate Logistics Hong Kong Limited Hong Kong dollar

Marketable Securities and Fai_2

Marketable Securities and Fair Value Measurements (Tables)	12 Months Ended
Marketable Securities and Fair Value Measurements (Tables)	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Marketable Securities	The following tables present details of the Company’s marketable securities as of December 31, 2019 and 2018 : December 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value (in thousands) Short-term: Investment securities $ 404,294 $ 20 $ (62 ) $ 404,252 Long-term: Investment securities 155,616 92 (18 ) 155,690 Total $ 559,910 $ 112 $ (80 ) $ 559,942 December 31, 2018 Amortized Cost Gross Unrealized Losses Estimated Fair Value (in thousands) Short-term: Investment securities $ 114,402 $ (124 ) $ 114,278 Long-term: Investment securities 6,603 (77 ) 6,526 Total $ 121,005 $ (201 ) $ 120,804
Schedule of the Fair Value of the Company's Financial Assets Measured at Fair Value on a Recurring Basis Based on the Three-tier Value Hierarchy	The following tables set forth the fair value of the Company's financial assets measured at fair value on a recurring basis as of December 31, 2019 and 2018 based on the three-tier value hierarchy described in Note 2 , Summary of Significant Accounting Policies : December 31, 2019 Level 1 Level 2 Level 3 Total (in thousands) Cash and cash equivalents: Cash $ 308,521 $ — $ — $ 308,521 Cash equivalents 274,232 — — 274,232 Total cash and cash equivalents 582,753 — — 582,753 Short-term investments: Investment securities — 404,252 — 404,252 Other non-current assets: Certificate of deposit 5,076 — — 5,076 Long-term: Investment securities — 155,690 — 155,690 Total $ 587,829 $ 559,942 $ — $ 1,147,771 December 31, 2018 Level 1 Level 2 Level 3 Total (in thousands) Cash and cash equivalents: Cash $ 134,375 $ — $ — $ 134,375 Cash equivalents 715,086 — — 715,086 Total cash and cash equivalents 849,461 — — 849,461 Short-term investments: Investment securities — 114,278 — 114,278 Other non-current assets Certificate of deposit 5,000 — — 5,000 Long-term: Investment securities — 6,526 — 6,526 Total $ 854,461 $ 120,804 $ — $ 975,265

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
Property and Equipment, net (Tables)	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Schedule of property and equipment, net	The following table summarizes property and equipment, net as of December 31, 2019 and 2018 : December 31, 2019 2018 (in thousands) Furniture and computer equipment $ 509,120 $ 339,761 Site and software development costs 297,252 172,653 Leasehold improvements 228,514 109,929 Construction in progress 45,503 90,104 Buildings (leased - Note 6) — 184,694 1,080,389 897,141 Less accumulated depreciation and amortization (455,845 ) (290,164 ) Property and equipment, net $ 624,544 $ 606,977

Leases (Tables)

Leases (Tables)	12 Months Ended
Leases (Tables)	Dec. 31, 2019
Leases [Abstract]
Schedule of Other Information Related to Leases	The following table presents other information related to leases: Year Ended December 31, 2019 (in thousands) Supplemental cash flows information: Cash payments included in operating cash flows from lease arrangements $ 109,163 Right-of-use assets obtained in exchange for lease obligations $ 301,053 December 31, 2019 Additional lease information: Weighted average remaining lease term 10 years Weighted average discount rate 6.7 %
Schedule of Future Minimum Lease Payments	Future minimum lease payments under non-cancellable leases as of December 31, 2019 were as follows: Amount (in thousands) 2020 $ 143,398 2021 150,025 2022 141,866 2023 137,276 2024 134,724 Thereafter 526,671 Total future minimum lease payments 1,233,960 Less: Imputed interest (320,254 ) Total $ 913,706
Operating Leases, Balance Sheet Items	The following table presents total operating leases: December 31, 2019 (in thousands) Balance sheet line item: Other current liabilities 91,104 Operating lease liabilities 822,602 Total operating leases $ 913,706

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities (Tables)	12 Months Ended
	Dec. 31, 2019
Payables and Accruals [Abstract]
Components of Prepaid Expenses and Other Current Assets	The following table presents the components of selected balance sheet items as of December 31, 2019 and 2018 : December 31, 2019 2018 (in thousands) Prepaid expenses and other current assets: Deferred costs in transit $ 104,947 $ 83,652 Prepaid assets 46,177 31,003 Supplier receivable 28,245 31,842 Supplier credits receivable 20,083 17,667 Other current assets 29,269 31,266 Total prepaid expenses and other current assets $ 228,721 $ 195,430
Components of Accrued Expenses	December 31, 2019 2018 (in thousands) Accrued expenses: Employee compensation and related benefits $ 141,922 $ 91,914 Advertising 71,597 67,259 Credit card 13,943 16,740 Accrued property, plant and equipment 18,348 13,955 Audit, legal and professional fees 4,018 1,703 Other accrued expenses 49,090 21,426 Total accrued expenses $ 298,918 $ 212,997
Components of Other Liabilities	December 31, 2019 2018 (in thousands) Other current liabilities: Short term lease liability (1) $ 91,104 $ — Sales tax payable 62,173 49,989 Sales return reserve 38,042 35,707 Other current liabilities 45,544 42,299 Total current other liabilities $ 236,863 $ 127,995 December 31, 2019 2018 (in thousands) Other liabilities: Construction costs under build-to-suit leases (1) $ — $ 60,089 Deferred rent (1) — 99,746 Other liabilities 6,940 553 Total other liabilities $ 6,940 $ 160,388 (1) On January 1, 2019 the Company adopted ASU 2016-02. Refer to Note 2 , Summary of Significant Accounting Policies

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
Commitments and Contingencies (Tables)	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Summary of Principal Purchase Obligations	The following table summarizes our purchase obligations as of December 31, 2019 : Payments Due During Year Ended December 31, 2020 2021 2022 2023 2024 Thereafter Total (in thousands) Purchase Obligations $ 21,206 $ 25,759 $ 15,653 $ 2,047 $ 1,195 $ — $ 65,860

Equity-Based Compensation (Tabl

Equity-Based Compensation (Tables)	12 Months Ended
Equity-Based Compensation (Tables)	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Activity Relating to Stock Options	The following table presents activity relating to stock options for the year ended December 31, 2019 : Shares Weighted-Average Exercise Price Weighted-Average Remaining Contractual Term (Years) Outstanding at December 31, 2018 79,802 $ 3.05 2.5 Options exercised (35,996 ) $ 3.11 Options expired (200 ) $ 3.42 Outstanding and exercisable at December 31, 2019 43,606 $ 3.00 1.5
Activity Relating to Restricted Stock	The following table presents activity relating to restricted common stock for the year ended December 31, 2019 : Shares Weighted-Average Grant Date Fair Value Unvested at December 31, 2018 20,000 $ 44.34 Restricted stock vested (20,000 ) $ 44.34 Unvested as of December 31, 2019 — $ —
Activity Relating to Restricted Stock Units	The following table presents activity relating to RSUs for the year ended December 31, 2019 : Shares Weighted-Average Grant Date Fair Value Unvested at December 31, 2018 7,970,959 $ 72.43 RSUs granted 4,420,948 $ 120.29 RSUs vested (2,835,165 ) $ 74.47 RSUs forfeited/canceled (1,444,006 ) $ 84.51 Outstanding as of December 31, 2019 8,112,736 $ 95.69

Segment and Geographic Inform_2

Segment and Geographic Information (Tables)	12 Months Ended
Segment and Geographic Information (Tables)	Dec. 31, 2019
Segment Reporting [Abstract]
Activity Related to Net Revenue and Adjusted EBITDA by Segment	The following tables present net revenues and Adjusted EBITDA attributable to the Company’s reportable segments for the periods presented: Year Ended December 31, 2019 2018 2017 (in thousands) U.S. net revenue $ 7,764,831 $ 5,813,070 $ 4,153,057 International net revenue 1,362,226 966,104 567,838 Total net revenue $ 9,127,057 $ 6,779,174 $ 4,720,895 Year Ended December 31, 2019 2018 2017 (in thousands) Adjusted EBITDA: U.S. $ (179,010 ) $ (19,049 ) $ 35,888 International (317,534 ) (195,937 ) (102,921 ) Total reportable segments Adjusted EBITDA (496,544 ) (214,986 ) (67,033 ) Less: reconciling items (1) (488,040 ) (289,094 ) (177,581 ) Net loss $ (984,584 ) $ (504,080 ) $ (244,614 ) (1) The following adjustments are made to reconcile total reportable segments Adjusted EBITDA to consolidated net loss: Year Ended December 31, 2019 2018 2017 (in thousands) Depreciation and amortization (1) $ 192,419 $ 123,542 $ 87,020 Equity-based compensation and related taxes 240,978 136,415 72,626 Interest (expense), net 54,514 28,560 9,433 Other (income) expense, net (2,881 ) 204 (758 ) Provision for income taxes, net 3,010 2,037 486 Other (1) — (1,664 ) 8,774 Total reconciling items $ 488,040 $ 289,094 $ 177,581 (1) The Company recorded $9.6 million of one-time charges in the year ended December 31, 2017 in selling, operations, technology, general and administrative in the Consolidated Statements of Operations related to a warehouse the Company vacated in July 2017. Of the $9.6 million charges, $8.8 million was included in other and related primarily to the excess of the Company's estimated future remaining lease commitments through 2023 over its expected sublease income over the same period, and $0.8 million was included in depreciation and amortization related to accelerated depreciation of leasehold improvements in the warehouse. In the year ended December 31, 2018, the Company terminated the lease and recognized in Other a $1.7 million one-time gain related to the difference in the expected future net lease commitments and the actual costs incurred to terminate the lease. The gain was recognized in selling, operations, technology, general and administrative in the Consolidated Statements of Operations
Schedule of Long-lived Assets by Geographic Area	The following table presents long-lived assets attributable to the Company's reportable segments reconciled to the amounts: Year Ended December 31, 2019 2018 (in thousands) Geographic long-lived assets: U.S. $ 731,963 $ 470,260 International 144,803 31,303 Total reportable segment long-lived assets 876,766 501,563 Plus: reconciling corporate long-lived assets 511,178 105,414 Total long-lived assets $ 1,387,944 $ 606,977
Assets By Segment	The following table presents total assets attributable to the Company's reportable segments reconciled to consolidated amounts: Year Ended December 31, 2019 2018 (in thousands) Assets by segment: U.S. $ 1,076,733 $ 738,600 International 190,167 55,159 Total reportable segment assets 1,266,900 793,759 Plus: reconciling corporate assets 1,686,148 1,097,091 Total assets $ 2,953,048 $ 1,890,850

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
Income Taxes (Tables)	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Schedule of Income Tax Expense (Benefit) From Continuing Operation	The components of the provision for income taxes, net for the years ended December 31, 2019 , 2018 and 2017 is presented below: Year ended December 31, 2019 Current Deferred Total (in thousands) Federal $ — $ — $ — State 874 — 874 Foreign 2,136 — 2,136 $ 3,010 $ — $ 3,010 Year ended December 31, 2018 Current Deferred Total (in thousands) Federal $ — $ (86 ) $ (86 ) State 744 (24 ) 720 Foreign 78 1,325 1,403 $ 822 $ 1,215 $ 2,037 Year ended December 31, 2017 Current Deferred Total (in thousands) Federal $ — $ (31 ) $ (31 ) State 540 8 548 Foreign 942 (973 ) (31 ) $ 1,482 $ (996 ) $ 486
Schedule of Difference in the Company's Expected Tax Expense (Benefit) as Computed by Applying the U.S. Federal Corporate Rate to Income Before Tax Expense (Benefit), and Actual Tax is Reconciled	The actual provision for income taxes, net differs from the expected provision for income taxes computed at the U.S. Federal statutory tax rate of 21% due to the following: Year Ended December 31, 2019 2018 2017 (in thousands) Provision for income taxes at the federal statutory rate $ (206,131 ) $ (105,429 ) $ (85,445 ) State income tax expense, net of federal benefit (40,136 ) (22,584 ) (11,432 ) Foreign tax rate differential 24,346 14,976 23,179 Non-deductible equity-based compensation expense 6,604 3,267 1,080 Windfall benefits from equity-based compensation (28,915 ) (29,003 ) (24,168 ) Change in valuation allowance 236,863 119,370 24,209 Change in tax rate (2,293 ) 197 71,919 Limitation on officer's compensation 6,509 5,283 — Debt integration termination — 9,236 — Other 6,163 6,724 1,144 Provision for income taxes, net $ 3,010 $ 2,037 $ 486
Schedule of Components of Income Tax Expense (Benefit) Determined by Tax Jurisdiction	The components of loss before income taxes determined by tax jurisdiction, are as follows: Year Ended December 31, 2019 2018 2017 (in thousands) U.S. $ (699,347 ) $ (327,356 ) $ (143,800 ) Foreign (282,227 ) (174,687 ) (100,328 ) Total $ (981,574 ) $ (502,043 ) $ (244,128 )
Schedule of Tax Effects of Temporary Differences that Give Rise to Significant Portions of Deferred Tax Assets and Liabilities	The tax effects of temporary differences that give rise to significant portions of deferred tax assets and liabilities for the periods presented are as follows: December 31, 2019 2018 (in thousands) Deferred tax assets: Accounts receivable $ 5,909 $ 2,400 Inventories 871 495 Net operating loss carry-forwards 470,199 262,481 Equity-based compensation expense 14,929 9,733 Intangibles 10,675 12,526 Accrued payroll 21,395 13,900 Accrued expenses and reserves 14,539 12,339 Charitable contributions 872 657 Leases 239,259 90,868 Gross deferred tax assets 778,648 405,399 Less: Valuation allowance (412,898 ) (247,454 ) Net deferred tax assets $ 365,750 $ 157,945 Deferred tax liabilities: Prepaid expenses $ (5,930 ) $ (3,030 ) Capitalized technology (27,354 ) (15,427 ) Property and equipment (14,191 ) (73,189 ) Operating lease right-of-use asset (197,680 ) — Goodwill (40 ) (133 ) Convertible debt (120,447 ) (66,166 ) Other (108 ) — Total deferred tax liabilities (365,750 ) (157,945 ) Net deferred tax assets (liabilities) — — Non-current net deferred tax assets (liabilities) $ — $ —

Convertible Debt (Tables)

Convertible Debt (Tables)	12 Months Ended
Convertible Debt (Tables)	Dec. 31, 2019
Debt Disclosure [Abstract]
Schedule of Outstanding Principal and Carrying Value	The following table presents the outstanding principal amount and carrying value of the Notes as of the date presented: December 31, 2019 December 31, 2018 2017 Notes 2018 Notes 2019 Notes 2017 Notes 2018 Notes (in thousands) Principal amounts: Principal $ 431,250 $ 575,000 $ 948,750 $ 431,250 $ 575,000 Unamortized debt discount (59,830 ) (161,275 ) (277,700 ) (79,911 ) (187,435 ) Net carrying amount $ 371,420 $ 413,725 $ 671,050 $ 351,339 $ 387,565
Schedule Of Interest Expense Related to Notes	The following table presents total interest expense recognized related to the Notes: December 31, 2019 December 31, 2018 December 31, 2017 2017 Notes 2018 Notes 2019 Notes 2017 Notes 2018 Notes 2017 Notes (in thousands) Contractual interest expense $ 1,617 $ 6,469 $ 3,342 $ 1,617 $ 1,455 $ 1,491 Interest cost related to amortization of the debt discount 20,080 26,161 12,205 18,910 2,207 4,338 Total interest expense $ 21,697 $ 32,630 $ 15,547 $ 20,527 $ 3,662 $ 5,829

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
Net Loss per Share (Tables)	Dec. 31, 2019
Earnings Per Share [Abstract]
Schedule of Calculation of Basic and Diluted Net Loss Per Share	The following table presents the calculation of basic and diluted net loss per share: Year Ended December 31, 2019 2018 2017 (in thousands, except per share data) Net loss $ (984,584 ) $ (504,080 ) $ (244,614 ) Weighted average common shares used for basic and diluted net loss per share computation 92,200 89,472 86,983 Net loss per common share: Basic and Diluted $ (10.68 ) $ (5.63 ) $ (2.81 )

Description of Business (Detail

Description of Business (Details) supplier in Thousands, product in Millions	12 Months Ended
	Dec. 31, 2019productsupplier
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of products offered (over) \| product	18,000,000
Number of suppliers providing products offered (over) \| supplier	12

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details)	12 Months Ended
Summary of Significant Accounting Policies (Details)	Dec. 31, 2019USD ($)common_stock_class	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Jan. 01, 2019USD ($)	Jan. 01, 2018USD ($)	Jan. 01, 2017USD ($)
Concentrations of Credit Risk
Cash and cash equivalents and short-term investments held in banks located outside the U.S.	$ 48,200,000	$ 129,500,000
Goods In-Transit
Goods in-transit	104,900,000	83,700,000
Property, plant and equipment
Property and equipment, net	624,544,000	606,977,000		$ 379,268,000
Amortization expense	81,600,000	51,300,000	$ 34,500,000
Long-Lived Assets
Impairment of long-lived assets or identifiable intangibles	0	0	0
Revenue Recognition
Sales return reserve	38,042,000	35,707,000
Allowance for sales returns	38,000,000	35,700,000
Contract liabilities	167,641,000	148,057,000
Cumulative-effect adjustment to reduce Accumulated deficit	2,065,423,000	1,082,689,000		$ 1,080,839,000
Advertising Costs
Advertising expense	1,095,840,000	774,189,000	549,959,000
Prepaid advertising costs	3,800,000	700,000
Merchant Processing Fees
Merchant processing fees	$ 179,700,000	133,400,000	$ 88,700,000
Earnings Per Share
Number of classes of common stock \| common_stock_class	2
ASU 2014-09
Equity-based Compensation
Cumulative-effect adjustment for adoption of ASU					$ (4,657,000)
ASU 2014-09 \| Difference between Revenue Guidance in Effect before and after Topic 606
Revenue Recognition
Cumulative-effect adjustment to reduce Accumulated deficit					4,700,000
ASU 2014-09 \| Retained Earnings
Equity-based Compensation
Cumulative-effect adjustment for adoption of ASU					$ (4,657,000)
ASU 2016-09
Equity-based Compensation
Cumulative-effect adjustment for adoption of ASU						$ 0
ASU 2016-09 \| Retained Earnings
Equity-based Compensation
Cumulative-effect adjustment for adoption of ASU						$ 8,712,000
Deferred Revenue
Revenue Recognition
Contract liabilities	$ 167,600,000	148,100,000
Revenue recognized	126,000,000
Other Current Liabilities
Revenue Recognition
Contract liabilities	4,600,000	3,100,000
Revenue recognized	$ 3,100,000
Other Long-term Investments \| Minimum
Marketable Securities [Abstract]
Marketable securities, term	1 year
Other Long-term Investments \| Maximum
Marketable Securities [Abstract]
Marketable securities, term	2 years
Furniture and computer equipment \| Minimum
Property, plant and equipment
Estimated useful lives	3 years
Furniture and computer equipment \| Maximum
Property, plant and equipment
Estimated useful lives	7 years
Site and software development costs
Property, plant and equipment
Estimated useful lives	2 years
Property and equipment, net	$ 123,500,000	$ 62,900,000
Building
Property, plant and equipment
Estimated useful lives	30 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Adoption of ASU 2016-02 (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and equipment, net	$ 624,544	$ 379,268	$ 606,977
Operating lease right-of-use assets	763,400	524,082
Other noncurrent assets	13,467	12,012	18,826
Other liabilities	6,940	4,392	160,388
Other current liabilities	236,863	118,371	127,995
Lease financing obligations, net of current portion			183,056
Lease financing obligation, net of current portion	0	0
Operating lease liabilities	913,706	636,385
Accumulated deficit	$ (2,065,423)	(1,080,839)	$ (1,082,689)
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and equipment, net		(227,709)
Operating lease right-of-use assets		524,082
Other noncurrent assets		(6,814)
Other liabilities		(155,996)
Other current liabilities		(9,624)
Lease financing obligations, net of current portion		(183,056)
Operating lease liabilities		636,385
Accumulated deficit		$ 1,850

Marketable Securities and Fai_3

Marketable Securities and Fair Value Measurements - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities, available-for-sale	$ 559,942,000	$ 120,804,000
Other-than-temporary impairment loss	0	0	$ 0
Realized gains (losses)	0	0	$ 0
Other Long-term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities, available-for-sale	$ 155,690,000	$ 6,526,000
Minimum \| Other Long-term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities, term	1 year
Maximum \| Other Long-term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities, term	2 years

Marketable Securities and Fai_4

Marketable Securities and Fair Value Measurements - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair value measurements
Amortized Cost	$ 559,910	$ 121,005
Gross Unrealized Gains	112
Gross Unrealized Losses	(80)	(201)
Estimated Fair Value	559,942	120,804
Short-term investments
Fair value measurements
Amortized Cost	404,294	114,402
Gross Unrealized Gains	20
Gross Unrealized Losses	(62)	(124)
Estimated Fair Value	404,252	114,278
Long-term investments
Fair value measurements
Amortized Cost	155,616	6,603
Gross Unrealized Gains	92
Gross Unrealized Losses	(18)	(77)
Estimated Fair Value	$ 155,690	$ 6,526

Marketable Securities and Fai_5

Marketable Securities and Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair value measurements
Cash equivalents	$ 582,753	$ 849,461
Total	1,147,771	975,265
Short-term Investments
Fair value measurements
Short term investments	404,252	114,278
Certificate of deposit
Fair value measurements
Other non-current assets	5,076	5,000
Investment securities
Fair value measurements
Long term investments	155,690	6,526
Cash
Fair value measurements
Cash equivalents	308,521	134,375
Cash equivalents
Fair value measurements
Cash equivalents	274,232	715,086
Level 1
Fair value measurements
Cash equivalents	582,753	849,461
Total	587,829	854,461
Level 1 \| Short-term Investments
Fair value measurements
Short term investments	0	0
Level 1 \| Certificate of deposit
Fair value measurements
Other non-current assets	5,076	5,000
Level 1 \| Investment securities
Fair value measurements
Long term investments	0	0
Level 1 \| Cash
Fair value measurements
Cash equivalents	308,521	134,375
Level 1 \| Cash equivalents
Fair value measurements
Cash equivalents	274,232	715,086
Level 2
Fair value measurements
Cash equivalents	0	0
Total	559,942	120,804
Level 2 \| Short-term Investments
Fair value measurements
Short term investments	404,252	114,278
Level 2 \| Certificate of deposit
Fair value measurements
Other non-current assets	0	0
Level 2 \| Investment securities
Fair value measurements
Long term investments	155,690	6,526
Level 2 \| Cash
Fair value measurements
Cash equivalents	0	0
Level 2 \| Cash equivalents
Fair value measurements
Cash equivalents	0	0
Level 3
Fair value measurements
Cash equivalents	0	0
Total	0	0
Level 3 \| Short-term Investments
Fair value measurements
Short term investments	0	0
Level 3 \| Certificate of deposit
Fair value measurements
Other non-current assets	0	0
Level 3 \| Investment securities
Fair value measurements
Long term investments	0	0
Level 3 \| Cash
Fair value measurements
Cash equivalents	0	0
Level 3 \| Cash equivalents
Fair value measurements
Cash equivalents	$ 0	$ 0

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets	$ 18.4	$ 0.5
Amortization expense related to intangible assets	0.8	0.9	$ 1.1
Goodwill	$ 0.4	$ 2.1

Property and Equipment, net (De

Property and Equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,080,389	$ 897,141
Less accumulated depreciation and amortization	(455,845)	(290,164)
Property and equipment, net	624,544	606,977		$ 379,268
Depreciation and amortization	191,600	122,600	$ 85,900
Furniture and computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	509,120	339,761
Site and software development costs
Property, Plant and Equipment [Line Items]
Property and equipment, gross	297,252	172,653
Property and equipment, net	123,500	62,900
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	228,514	109,929
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	45,503	90,104
Building
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 0	$ 184,694

Leases - Narrative (Details)

Leases - Narrative (Details) $ in Thousands	12 Months Ended
Leases - Narrative (Details) $ in Thousands	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)lease	Dec. 31, 2017USD ($)	Jan. 01, 2019USD ($)
Lessee, Lease, Description [Line Items]
Operating lease expense	$ 122,000
Operating leases expense		$ 66,700	$ 48,400
Operating lease, lease not yet commenced, expense	342,300
Letters of credit outstanding, amount	46,700	$ 26,800
Lease financing obligation, net of current portion	$ 0			$ 0
Property Lease Guarantee
Lessee, Lease, Description [Line Items]
Warehouse lease arrangements \| lease		3
Lease financing obligation, net of current portion		$ 101,000
Warehouse Lease Arrangement \| Property Lease Guarantee
Lessee, Lease, Description [Line Items]
Letters of credit outstanding, amount		$ 2,500
Minimum
Lessee, Lease, Description [Line Items]
Operating leases, Lease not yet commenced, terms	5 years
Maximum
Lessee, Lease, Description [Line Items]
Operating leases, Lease not yet commenced, terms	15 years

Leases - Other Information Rela

Leases - Other Information Related to Leases (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Supplemental cash flows information:
Cash payments included in operating cash flows from lease arrangements	$ 109,163
Right-of-use assets obtained in exchange for lease obligations	$ 301,053
Additional lease information:
Weighted average remaining lease term	10 years
Weighted average discount rate	6.70%

Leases - Future Minimum Lease P

Leases - Future Minimum Lease Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019
Leases [Abstract]
2020	$ 143,398
2021	150,025
2022	141,866
2023	137,276
2024	134,724
Thereafter	526,671
Total future minimum lease payments	1,233,960
Less: Imputed interest	(320,254)
Total	$ 913,706	$ 636,385

Leases - Operating Leases Balan

Leases - Operating Leases Balance Sheet Location (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019
Leases [Abstract]
Other current liabilities	$ 91,104
Operating lease liabilities	822,602
Total operating leases	$ 913,706	$ 636,385

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities - Components of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Prepaid expenses and other current assets:
Deferred costs in transit	$ 104,947	$ 83,652
Prepaid assets	46,177	31,003
Supplier receivable	28,245	31,842
Supplier credits receivable	20,083	17,667
Other current assets	29,269	31,266
Total prepaid expenses and other current assets	$ 228,721	$ 195,430

Prepaid Expenses and Other Cu_4

Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities - Components of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued expenses:
Employee compensation and related benefits	$ 141,922	$ 91,914
Advertising	71,597	67,259
Credit card	13,943	16,740
Accrued property, plant and equipment	18,348	13,955
Audit, legal and professional fees	4,018	1,703
Other accrued expenses	49,090	21,426
Total accrued expenses	$ 298,918	$ 212,997

Prepaid Expenses and Other Cu_5

Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities - Components of Other Current Liabilities and Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Other current liabilities:
Short term lease liability	$ 91,104
Sales tax payable	62,173		$ 49,989
Sales return reserve	38,042		35,707
Other current liabilities	45,544		42,299
Total current other liabilities	236,863	$ 118,371	127,995
Other liabilities:
Construction costs under build-to-suit leases	0		60,089
Deferred rent	0		99,746
Other liabilities	6,940		553
Total other liabilities	$ 6,940	$ 4,392	$ 160,388

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) $ in Millions	Jan. 16, 2019officerplaintiff	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Commitments and Contingencies Disclosure [Abstract]
Letters of credit outstanding, amount \| $		$ 46.7	$ 26.8
Number of officers of the Company \| officer	3
Complaints filed \| plaintiff	2

Commitments and Contingencies_2

Commitments and Contingencies - Unconditional Purchase Obligations (Details) $ in Thousands	Dec. 31, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
2020	$ 21,206
2021	25,759
2022	15,653
2023	2,047
2024	1,195
Thereafter	0
Total	$ 65,860

Employee Benefit Plans (Details

Employee Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
Employee Benefit Plans (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Benefit Plans
Age of the full-time employees qualified to participate in the defined contribution plan	21 years
Expense related to savings plan recognized	$ 27.9	$ 18.2	$ 9
Maximum
Employee Benefit Plans
Matching contribution by employer as a percentage of employee's considered compensation	4.00%

Equity-Based Compensation - Nar

Equity-Based Compensation - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
Stock options
Equity based compensation
Aggregate intrinsic value of stock options exercised	$ 4.8	$ 4.3
Aggregate intrinsic value of stock options outstanding	6.9
Aggregate intrinsic value of stock options exercisable	3.8
Restricted stock
Equity based compensation
Intrinsic value of stock vested and repurchased (less than $0.1 million)	1.8
Restricted stock units
Equity based compensation
Intrinsic value, of stock vested	358.6	$ 262.4
Aggregate intrinsic value of nonvested restricted common stock	733.1
Unrecognized share-based compensation	$ 705.5
Weighted average remaining vesting term for options expected to vest	1 year 4 months 24 days
2014 Plan
Equity based compensation
Number of shares authorized (in shares)	8,603,066
2014 Plan \| Subsequent Event
Equity based compensation
Number of shares available for future grant (in shares)			5,111,305

Equity-Based Compensation - Sum

Equity-Based Compensation - Summary of Activity Related to Stock Options (Details ) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Shares
Options expired (in shares)	(200)
Weighted-Average Exercise Price
Options expired (in dollars per share)	$ 3.42
Stock options
Shares
Outstanding at the beginning of the period (in shares)	79,802
Options exercised (in shares)	(35,996)
Outstanding and exercisable at the end of the period (in shares)	43,606	79,802
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 3.05
Options exercised (in dollars per share)	3.11
Outstanding and exercisable at the end of the period (in dollars per share)	$ 3	$ 3.05
Weighted-Average Remaining Contractual Term (Years)
Outstanding		2 years 6 months
Exercisable at the end of the period	1 year 6 months

Equity-Based Compensation - S_2

Equity-Based Compensation - Summary of Activity Related to Restricted Common Stock (Details) - Restricted stock	12 Months Ended
	Dec. 31, 2019$ / sharesshares
Restricted common stock
Unvested at the beginning of the period (in shares) \| shares	20,000
Restricted stock vested (in shares) \| shares	(20,000)
Unvested at the end of the period (in shares) \| shares	0
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) \| $ / shares	$ 44.34
Restricted stock vested (in dollars per share) \| $ / shares	44.34
Unvested at the end of the period (in dollars per share) \| $ / shares	$ 0

Equity-Based Compensation - S_3

Equity-Based Compensation - Summary of Activity Relating to RSU's (Details) - Restricted stock units	12 Months Ended
	Dec. 31, 2019$ / sharesshares
Shares
Unvested at the beginning of the period (in shares) \| shares	7,970,959
RSU's granted (in shares) \| shares	4,420,948
RSUs vested (in shares) \| shares	(2,835,165)
RSUs forfeited/cancelled (in shares) \| shares	(1,444,006)
Unvested at the end of the period (in shares) \| shares	8,112,736
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) \| $ / shares	$ 72.43
RSUs granted (in dollars per share) \| $ / shares	120.29
RSUs vested (in dollars per share) \| $ / shares	74.47
RSUs forfeited/cancelled (in dollars per share) \| $ / shares	84.51
Unvested at the end of the period (in dollars per share) \| $ / shares	$ 95.69

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit) (Details)	12 Months Ended
Stockholders' Equity (Deficit) (Details)	Dec. 31, 2019vote$ / sharesshares	Dec. 31, 2018$ / sharesshares
Stockholders' Equity
Preferred stock, shares authorized (in shares)	10,000,000
Preferred stock, par value (in dollars per share) \| $ / shares	$ 0.001
Preferred stock, shares issued (in shares)	0
Preferred stock, shares outstanding (in shares)	0
Class A common stock
Common stock
Common stock, shares authorized (in shares)	500,000,000	500,000,000
Common stock, par value (in dollars per share) \| $ / shares	$ 0.001	$ 0.001
Common stock, shares outstanding (in shares)	66,642,611	62,329,701
Number of votes each holder is entitled \| vote	1
Class B common stock
Common stock
Common stock, shares authorized (in shares)	164,000,000	164,000,000
Common stock, par value (in dollars per share) \| $ / shares	$ 0.001	$ 0.001
Common stock, shares outstanding (in shares)	26,957,815	28,417,882
Number of votes each holder is entitled \| vote	10
Conversion ratio	1
Conversion ratio upon transfer	1
Number of shares converted into Class A shares (in shares)	55,081,618
Class B common stock \| Minimum
Common stock
Percentage of outstanding shares of class B common stock that shall convert automatically in event of affirmative vote or written consent of holders for automatic conversion	66.67%
Class B common stock \| Maximum
Common stock
Aggregate number of shares outstanding Class A common stock and Class B common stock that will automatically convert (less than)	10.00%

Segment and Geographic Inform_3

Segment and Geographic Information - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net revenue	$ 9,127,057	$ 6,779,174	$ 4,720,895
U.S.
Segment Reporting Information [Line Items]
Net revenue	$ 7,764,831	5,813,070	4,153,057
Other
Segment Reporting Information [Line Items]
Net revenue			$ (77,700)
Other \| U.S.
Segment Reporting Information [Line Items]
Net revenue		$ (61,100)

Segment and Geographic Inform_4

Segment and Geographic Information - Net Revenues and Adjusted EBITDA (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total net revenue		$ 9,127,057	$ 6,779,174	$ 4,720,895
Adjusted EBITDA		(496,544)	(214,986)	(67,033)
Less: reconciling items		(488,040)	(289,094)	(177,581)
Net loss		(984,584)	(504,080)	(244,614)
Depreciation and amortization		192,419	123,542	87,020
Equity-based compensation and related taxes		240,978	136,415	72,626
Interest (expense), net		54,514	28,560	9,433
Other (income) expense, net		(2,881)	204	(758)
Provision for income taxes, net		3,010	2,037	486
Other		0	(1,664)	8,774
Total reconciling items		488,040	289,094	177,581
Selling, operations, technology, general and administrative		1,624,706	1,025,767	634,801
Leasehold improvements
Segment Reporting Information [Line Items]
Depreciation and amortization				800
Warehouse vacated
Segment Reporting Information [Line Items]
Other	$ 1,700			8,800
Selling, operations, technology, general and administrative				9,600
U.S.
Segment Reporting Information [Line Items]
Total net revenue		7,764,831	5,813,070	4,153,057
Adjusted EBITDA		(179,010)	(19,049)	35,888
International
Segment Reporting Information [Line Items]
Total net revenue		1,362,226	966,104	567,838
Adjusted EBITDA		$ (317,534)	$ (195,937)	$ (102,921)

Segment and Geographic Inform_5

Segment and Geographic Information - Long-lived Assets by Segment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Geographic long-lived assets:
Long-lived assets	$ 1,387,944	$ 606,977
Plus: reconciling corporate long-lived assets
Geographic long-lived assets:
Long-lived assets	511,178	105,414
Operating Segments \| U.S.
Geographic long-lived assets:
Long-lived assets	731,963	470,260
Operating Segments \| International
Geographic long-lived assets:
Long-lived assets	144,803	31,303
Total reportable segment long-lived assets
Geographic long-lived assets:
Long-lived assets	$ 876,766	$ 501,563

Segment and Geographic Inform_6

Segment and Geographic Information - Assets by Segment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets by segment:
Total assets	$ 2,953,048	$ 1,890,850
Plus: reconciling corporate assets
Assets by segment:
Total assets	1,686,148	1,097,091
Operating Segments \| U.S.
Assets by segment:
Total assets	1,076,733	738,600
Operating Segments \| International
Assets by segment:
Total assets	190,167	55,159
Segment Reconciling Items
Assets by segment:
Total assets	$ 1,266,900	$ 793,759

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current income tax expense (benefit)
Federal	$ 0	$ 0	$ 0
State	874	744	540
Foreign	2,136	78	942
Current income tax expense (benefit)	3,010	822	1,482
Deferred income tax expense (benefit)
Federal	0	(86)	(31)
State	0	(24)	8
Foreign	0	1,325	(973)
Deferred income tax expense (benefit)	0	1,215	(996)
Income tax expense (benefit)
Federal, Total	0	(86)	(31)
State, Total	874	720	548
Foreign, Total	2,136	1,403	(31)
Net income tax expense	3,010	2,037	486
Reconciliation of the U.S. federal corporate rate to to income before tax expense (benefit), and actual tax
Provision for income taxes at the federal statutory rate	(206,131)	(105,429)	(85,445)
Decrease in income taxes resulting from:
State income tax expense, net of federal benefit	(40,136)	(22,584)	(11,432)
Foreign tax rate differential	24,346	14,976	23,179
Non-deductible equity-based compensation expense	6,604	3,267	1,080
Windfall benefits from equity-based compensation	(28,915)	(29,003)	(24,168)
Change in valuation allowance	236,863	119,370	24,209
Change in tax rate	(2,293)	197	71,919
Limitation on officer's compensation	6,509	5,283	0
Debt integration termination	0	9,236	0
Other	6,163	6,724	1,144
Net income tax expense	$ 3,010	2,037	486
Income (loss) from continuing operations:
Effective tax rate (almost), percent	0.00%
U.S.	$ (699,347)	(327,356)	(143,800)
Foreign	(282,227)	(174,687)	(100,328)
Loss before income taxes	(981,574)	(502,043)	$ (244,128)
Deferred tax assets:
Accounts receivable	5,909	2,400
Inventories	871	495
Net operating loss carry-forwards	470,199	262,481
Equity-based compensation expense	14,929	9,733
Intangibles	10,675	12,526
Accrued payroll	21,395	13,900
Accrued expenses and reserves	14,539	12,339
Charitable contributions	872	657
Leases	239,259	90,868
Gross deferred tax assets	778,648	405,399
Less: Valuation allowance	(412,898)	(247,454)
Net deferred tax assets	365,750	157,945
Deferred tax liabilities:
Prepaid expenses	(5,930)	(3,030)
Capitalized technology	(27,354)	(15,427)
Property and equipment	(14,191)	(73,189)
Deferred Tax Liabilities, Right-of-Use Asset	(197,680)
Goodwill	(40)	(133)
Convertible debt	(120,447)	(66,166)
Other	(108)	0
Total deferred tax liabilities	(365,750)	(157,945)
Deferred tax assets	0	$ 0
Valuation allowance increase	165,400
Federal net operating loss carryforwards	1,400,000
State net operating loss carryforwards	1,300,000
Operating loss carryforwards, subject to expiration	427,400
Operating loss carryforwards not subject to expiration	1,000,000
Foreign operating loss carryforwards	686,900
Undistributed earnings of the entity's foreign subsidiaries	7,300
Foreign Tax Authority
Deferred tax liabilities:
Operating loss carryforwards, subject to expiration	$ 11,800

Credit Agreement (Details)

Credit Agreement (Details) - USD ($)	Feb. 19, 2019	Dec. 31, 2019	Feb. 21, 2019	Dec. 31, 2018
Credit agreement
Borrowing under credit agreement		$ 0		$ 0
Revolving Credit Facility
Credit agreement
Maximum borrowing capacity			$ 165,000,000
Right to increase			50,000,000
Borrower Obligations, capital stock of first-tier foreign subsidiaries, percent	65.00%
Revolving Credit Facility \| Federal Funds Effective Swap Rate
Credit agreement
Basis spread	0.50%
Revolving Credit Facility \| Eurodollar
Credit agreement
Basis spread	1.00%
Applicable margin	0.0175
Revolving Credit Facility \| Base Rate
Credit agreement
Applicable margin	0.0075
Revolving Credit Facility \| Letter of Credit Sublimit
Credit agreement
Maximum borrowing capacity			100,000,000
Revolving Credit Facility \| Revolver Swing Line Supplement
Credit agreement
Maximum borrowing capacity			$ 15,000,000

Convertible Debt (Details)

Convertible Debt (Details)	Aug. 19, 2019USD ($)day$ / shares	Aug. 14, 2019$ / shares	Nov. 19, 2018USD ($)day$ / shares	Sep. 15, 2017USD ($)day$ / shares	Sep. 11, 2017institution$ / shares	Nov. 30, 2018USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Debt Instrument [Line Items]
Number of Initial Purchasers \| institution					2
Proceeds from issuance of convertible notes, net of issuance costs							$ 935,146,000	$ 562,047,000	$ 420,449,000
Premiums paid for capped call confirmations							145,728,000	93,438,000	$ 44,160,000
Convertible Debt \| 2017 Notes
Debt Instrument [Line Items]
Principal				$ 431,250,000			431,250,000	431,250,000
Interest rate, stated percentage				0.375%
Proceeds from issuance of convertible notes, net of issuance costs				$ 420,400,000
Premiums paid for capped call confirmations				$ 44,200,000
Effective interest rate, percentage				6.00%
Equity component of Notes				$ 95,800,000
Fair value of Notes							469,500,000	465,000,000
Conversion ratio				0.00961
Conversion price (in usd per share) \| $ / shares				$ 104.06
Trading days (whether or not consecutively) \| day				20
Trading days (consecutive) \| day				30
Percentage of conversion stock price				130.00%
During number of business day period				5 days
Consecutive trading day period (after any)				10 days
Principal amount of Notes				$ 1,000
Measurement period percentage (less than)				98.00%
Minimum number of trading days immediately preceding the date on which notice or redemption provided \| day				1
Trading days immediately preceding date on which notice of redemption provided \| day				5
Redemption price, percentage of principal amount to be redeemed				100.00%
Default percentage of aggregate principal amount, of notes outstanding (not less than)				25.00%
Convertible Debt \| 2017 Notes \| Class A common stock
Debt Instrument [Line Items]
Conversion price (in usd per share) \| $ / shares				$ 104.06
Cap price per share (in usd per share) \| $ / shares					$ 154.16
Premium over last reported sale price, percentage					100.00%
Convertible Debt \| 2017 Notes \| Over-Allotment Option
Debt Instrument [Line Items]
Over-allotment option included in issuance of notes				$ 56,250,000
Convertible Debt \| 2018 Notes
Debt Instrument [Line Items]
Principal			$ 500,000,000			$ 575,000,000	575,000,000	575,000,000
Interest rate, stated percentage						1.125%
Over-allotment option included in issuance of notes			$ 75,000,000			$ 75,000,000
Proceeds from issuance of convertible notes, net of issuance costs						562,000,000
Premiums paid for capped call confirmations						$ 93,400,000
Effective interest rate, percentage						8.10%
Equity component of Notes						$ 181,500,000
Fair value of Notes							597,000,000	$ 580,200,000
Conversion ratio			0.008591
Trading days (whether or not consecutively) \| day			20
Trading days (consecutive) \| day			30
Percentage of conversion stock price			130.00%
During number of business day period			5 years
Consecutive trading day period (after any)			10 years
Principal amount of Notes			$ 1,000
Measurement period percentage (less than)			98.00%
Redemption price, percentage of principal amount to be redeemed			100.00%
Default percentage of aggregate principal amount, of notes outstanding (not less than)			25.00%
Convertible Debt \| 2018 Notes \| Class A common stock
Debt Instrument [Line Items]
Interest rate, stated percentage			1.125%
Conversion price (in usd per share) \| $ / shares			$ 116.40
Cap price per share (in usd per share) \| $ / shares			$ 219.63
Premium over last reported sale price, percentage			150.00%
Convertible Debt \| 2019 Notes
Debt Instrument [Line Items]
Principal	$ 948,750,000						948,750,000
Interest rate, stated percentage	1.00%
Over-allotment option included in issuance of notes	$ 123,750,000
Proceeds from issuance of convertible notes, net of issuance costs	935,100,000
Premiums paid for capped call confirmations	$ 145,700,000
Effective interest rate, percentage	6.40%
Equity component of Notes	$ 280,300,000
Fair value of Notes							$ 834,000,000
Conversion ratio	0.0067349
Trading days (consecutive) \| day	30
Percentage of conversion stock price	130.00%
During number of business day period	5 years
Consecutive trading day period (after any)	10 years
Principal amount of Notes	$ 1,000
Measurement period percentage (less than)	98.00%
Redemption price, percentage of principal amount to be redeemed	100.00%
Default percentage of aggregate principal amount, of notes outstanding (not less than)	25.00%
Convertible Debt \| 2019 Notes \| Class A common stock
Debt Instrument [Line Items]
Interest rate, stated percentage	1.00%
Conversion price (in usd per share) \| $ / shares	$ 148.48
Trading days (whether or not consecutively) \| day	20
Trading days (consecutive) \| day	30
Cap price per share (in usd per share) \| $ / shares	$ 280.15	$ 280.15
Premium over last reported sale price, percentage		150.00%

Convertible Debt - Schedule of

Convertible Debt - Schedule of Outstanding Principal and Carrying Value (Details) - USD ($)	Dec. 31, 2019	Aug. 19, 2019	Dec. 31, 2018	Nov. 30, 2018	Nov. 19, 2018	Sep. 15, 2017
Debt Instrument [Line Items]
Net carrying amount	$ 1,456,195,000		$ 738,904,000
Convertible Debt \| 2017 Notes
Debt Instrument [Line Items]
Principal	431,250,000		431,250,000			$ 431,250,000
Unamortized debt discount	(59,830,000)		(79,911,000)
Net carrying amount	371,420,000		351,339,000
Convertible Debt \| 2018 Notes
Debt Instrument [Line Items]
Principal	575,000,000		575,000,000	$ 575,000,000	$ 500,000,000
Unamortized debt discount	(161,275,000)		(187,435,000)
Net carrying amount	413,725,000		$ 387,565,000
Convertible Debt \| 2019 Notes
Debt Instrument [Line Items]
Principal	948,750,000	$ 948,750,000
Unamortized debt discount	(277,700,000)
Net carrying amount	$ 671,050,000

Convertible Debt - Schedule o_2

Convertible Debt - Schedule of Interest Expense Related to Notes (Details) - Convertible Debt - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
2017 Notes
Debt Instrument [Line Items]
Contractual interest expense	$ 1,617	$ 1,617	$ 1,491
Interest cost related to amortization of the debt discount	20,080	18,910	4,338
Total interest expense	21,697	20,527	$ 5,829
2018 Notes
Debt Instrument [Line Items]
Contractual interest expense	6,469	1,455
Interest cost related to amortization of the debt discount	26,161	2,207
Total interest expense	32,630	$ 3,662
2019 Notes
Debt Instrument [Line Items]
Contractual interest expense	3,342
Interest cost related to amortization of the debt discount	12,205
Total interest expense	$ 15,547

Net Loss per Share - Narrative

Net Loss per Share - Narrative (Details) shares in Millions	Aug. 19, 2019$ / sharesshares	Nov. 19, 2018$ / sharesshares	Sep. 15, 2017$ / sharesshares	Sep. 11, 2017$ / sharesshares	Dec. 31, 2019shares	Dec. 31, 2018shares	Dec. 31, 2017shares	Aug. 14, 2019$ / shares
Class of Stock [Line Items]
Allocation of undistributed earnings between stock classes, conversion ratio					1
2017 Notes \| Convertible Debt
Class of Stock [Line Items]
Conversion price (in usd per share) \| $ / shares			$ 104.06
2017 Notes \| Convertible Debt \| Class A common stock
Class of Stock [Line Items]
Conversion of stock, shares Issued (in shares) \| shares			4.1
Conversion price (in usd per share) \| $ / shares			$ 104.06
Cap price per share (in usd per share) \| $ / shares				$ 154.16
Capped Call Transaction, shares issued (in shares) \| shares				2.8
1.125% 2018 Senior Note Due 2024 \| Convertible Debt \| Class A common stock
Class of Stock [Line Items]
Conversion of stock, shares Issued (in shares) \| shares		4.9
Conversion price (in usd per share) \| $ / shares		$ 116.40
Cap price per share (in usd per share) \| $ / shares		$ 219.63
Capped Call Transaction, shares issued (in shares) \| shares		2.6
1.00% 2019 Convertible Senior Notes Due 2026 \| Convertible Debt \| Class A common stock
Class of Stock [Line Items]
Conversion of stock, shares Issued (in shares) \| shares	6.4
Conversion price (in usd per share) \| $ / shares	$ 148.48
Cap price per share (in usd per share) \| $ / shares	$ 280.15							$ 280.15
Capped Call Transaction, shares issued (in shares) \| shares	3.4
Common Stock Options, Restricted Stock Options And Restricted Stock Units
Class of Stock [Line Items]
Common stock outstanding that have been excluded from the computation of diluted net loss per share \| shares					8.2	8.1	7

Net Loss per Share - Calculatio

Net Loss per Share - Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net loss	$ (984,584)	$ (504,080)	$ (244,614)
Weighted average common shares used for basic and diluted net loss per share computation (in shares)	92,200	89,472	86,983
Earnings Per Share, Basic and Diluted [Abstract]
Basic and Diluted (in dollars per share)	$ (10.68)	$ (5.63)	$ (2.81)

Uncategorized Items - a2019-12x

Label	Element	Value
Accounting Standards Update 2016-02 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 1,850,000
Accounting Standards Update 2016-02 [Member] \| Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	1,850,000
Accounting Standards Update 2016-09 [Member] \| Additional Paid-in Capital [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 8,712,000