Alliance Resource Partners (ARLP) 10-K23 Feb 212020 FY Annual reportFinancial data

Company Profile

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2020	Feb. 23, 2021	Jun. 30, 2020
Document and Entity Information
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
Entity File Number	0-26823
Entity Registrant Name	ALLIANCE RESOURCE PARTNERS LP
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	73-1564280
Entity Address, Address Line One	1717 South Boulder Avenue, Suite 400
Entity Address, City or Town	Tulsa
Entity Address, State or Province	OK
Entity Address, Postal Zip Code	74119
City Area Code	918
Local Phone Number	295-7600
Title of 12(b) Security	Common Units
Trading Symbol	ARLP
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Common Units Outstanding		127,195,219
Entity Public Float			$ 343,214,355
Entity Central Index Key	0001086600
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
CURRENT ASSETS:
Cash and cash equivalents	$ 55,574	$ 36,482
Trade receivables	104,579	161,679
Other receivables	3,481	256
Inventories, net	56,407	101,305
Advance royalties	4,168	1,844
Prepaid expenses and other assets	21,565	18,019
Total current assets	245,774	319,585
PROPERTY, PLANT AND EQUIPMENT:
Property, plant and equipment, at cost	3,554,090	3,684,008
Less accumulated depreciation, depletion and amortization	(1,753,845)	(1,675,022)
Total property, plant and equipment, net	1,800,245	2,008,986
OTHER ASSETS:
Advance royalties	56,791	52,057
Equity method investments	27,268	28,529
Goodwill	4,373	136,399
Operating lease right-of-use assets	15,004	17,660
Other long-term assets	16,561	23,478
Total other assets	119,997	258,123
TOTAL ASSETS	2,166,016	2,586,694
CURRENT LIABILITIES:
Accounts payable	47,511	80,566
Accrued taxes other than income taxes	25,054	15,768
Accrued payroll and related expenses	28,524	36,575
Accrued interest	5,132	5,664
Workers' compensation and pneumoconiosis benefits	10,646	11,175
Current finance lease obligations	766	8,368
Current operating lease obligations	1,854	3,251
Other current liabilities	21,919	21,062
Current maturities, long-term debt, net	73,199	13,157
Total current liabilities	214,605	195,586
LONG-TERM LIABILITIES:
Long-term debt, excluding current maturities, net	519,421	768,194
Pneumoconiosis benefits	105,068	94,389
Accrued pension benefit	46,965	44,858
Workers' compensation	47,521	45,503
Non-current asset retirement obligations	121,487	133,018
Long-term finance lease obligations	1,458	2,224
Long-term operating lease obligations	13,078	14,316
Other liabilities	24,146	23,182
Total long-term liabilities	879,144	1,125,684
Total liabilities	1,093,749	1,321,270
ARLP Partners' Capital:
Limited Partners - Common Unitholders 127,195,219 and 126,915,597 units outstanding, respectively	1,148,565	1,331,482
Accumulated other comprehensive loss	(87,674)	(77,993)
Total ARLP Partners' Capital	1,060,891	1,253,489
Noncontrolling interest	11,376	11,935
Total Partners' Capital	1,072,267	1,265,424
TOTAL LIABILITIES AND PARTNERS' CAPITAL	$ 2,166,016	$ 2,586,694

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - shares	Dec. 31, 2020	Dec. 31, 2019
CONDENSED CONSOLIDATED BALANCE SHEETS
Common units outstanding	127,195,219	126,915,597

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SALES AND OPERATING REVENUES:
Revenues	$ 1,328,129	$ 1,961,720	$ 2,002,857
EXPENSES:
Operating expenses (excluding depreciation, depletion and amortization)	859,656	1,182,100	1,207,713
Outside coal purchases		23,357	1,466
General and administrative	59,806	72,997	68,298
Depreciation, depletion and amortization	313,387	309,075	280,225
Settlement gain			(80,000)
Asset impairments	24,977	15,190	40,483
Goodwill impairment	132,026	0	0
Total operating expenses	1,410,981	1,702,222	1,630,570
INCOME (LOSS) FROM OPERATIONS	(82,852)	259,498	372,287
Interest expense (net of interest capitalized of $1,325, $1,211 and $1,306, respectively)	(45,613)	(45,875)	(40,218)
Interest income	135	379	159
Equity method investment income	907	2,203	22,189
Equity securities income		12,906	15,696
Acquisition gain		177,043
Other income (expense)	(1,593)	561	(2,621)
INCOME (LOSS) BEFORE INCOME TAXES	(129,016)	406,715	367,492
INCOME TAX EXPENSE (BENEFIT)	35	(211)	22
NET INCOME (LOSS)	(129,051)	406,926	367,470
LESS: NET INCOME ATTRIBUTABLE TO NONCONTROLLING INTEREST	(169)	(7,512)	(866)
NET INCOME (LOSS) ATTRIBUTABLE TO ARLP	(129,220)	399,414	366,604
NET INCOME (LOSS) ATTRIBUTABLE TO ARLP
GENERAL PARTNER			1,560
LIMITED PARTNERS	$ (129,220)	$ 399,414	$ 365,044
EARNINGS PER LIMITED PARTNER UNIT - BASIC AND DILUTED	$ (1.02)	$ 3.07	$ 2.74
WEIGHTED-AVERAGE NUMBER OF UNITS OUTSTANDING - BASIC AND DILUTED	127,164,659	128,116,670	130,758,169
Coal sales
SALES AND OPERATING REVENUES:
Revenues	$ 1,232,272	$ 1,762,442	$ 1,844,808
Oil & gas royalties
SALES AND OPERATING REVENUES:
Revenues	42,912	51,735
Transportation revenues
SALES AND OPERATING REVENUES:
Revenues	21,129	99,503	112,385
EXPENSES:
Transportation expenses	21,129	99,503	112,385
Other revenues
SALES AND OPERATING REVENUES:
Revenues	$ 31,816	$ 48,040	$ 45,664

CONSOLIDATED STATEMENTS OF OP_2

CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS
Interest expense, interest capitalized	$ 1,325	$ 1,211	$ 1,306

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
NET INCOME (LOSS)	$ (129,051)	$ 406,926	$ 367,470
OTHER COMPREHENSIVE INCOME (LOSS):
Total defined benefit pension plan adjustments	(9,681)	(31,122)	5,069
OTHER COMPREHENSIVE INCOME (LOSS)	(9,681)	(31,122)	5,069
COMPREHENSIVE INCOME (LOSS)	(138,732)	375,804	372,539
Less: Comprehensive income attributable to noncontrolling interest	(169)	(7,512)	(866)
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO ARLP	(138,901)	368,292	371,673
Defined benefit pension plan
OTHER COMPREHENSIVE INCOME (LOSS):
Amortization of prior service cost	186	186	186
Net actuarial loss	(5,522)	(7,350)	(3,326)
Amortization of net actuarial gain loss	4,128	3,922	3,608
Total defined benefit pension plan adjustments	(1,208)	(3,242)	468
Pneumoconiosis benefits
OTHER COMPREHENSIVE INCOME (LOSS):
Net actuarial loss	(7,787)	(23,298)	4,599
Amortization of net actuarial gain loss	(686)	(4,582)	2
Total defined benefit pension plan adjustments	$ (8,473)	$ (27,880)	$ 4,601

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)	$ (129,051)	$ 406,926	$ 367,470
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	313,387	309,075	280,225
Non-cash compensation expense	3,345	11,934	12,114
Asset retirement obligations	4,033	4,087	3,926
Coal inventory adjustment to market	3,245	4,895	1,455
Equity investment income	(907)	(2,203)	(22,189)
Distributions from equity method investments	907	2,203	21,971
Income from equity securities paid-in-kind		(712)	(15,696)
Net loss (gain) on sale of property, plant and equipment	(5,850)	109	(1,285)
Asset impairment	24,977	15,190	40,483
Goodwill impairment	132,026	0	0
Acquisition gain, net		(177,043)
Cash received on redemption of equity securities in excess of investment		(11,482)
Valuation allowance of deferred tax assets	1,151	(413)	(1,560)
Other	6,631	5,677	3,171
Changes in operating assets and liabilities:
Trade receivables	56,172	20,841	6,757
Other receivables	(3,225)	3,726	(249)
Inventories, net	30,522	(35,082)	(747)
Prepaid expenses and other assets	(2,514)	6,136	7,387
Advance royalties	(7,690)	(9,876)	(8,782)
Accounts payable	(24,282)	(17,671)	(813)
Accrued taxes other than income taxes	9,286	(994)	(3,614)
Accrued payroll and related benefits	(8,051)	(6,538)	7,362
Pneumoconiosis benefits	2,340	(2,292)	1,837
Workers' compensation	1,355	3,845	(4,900)
Other	(7,162)	(15,443)	22
Total net adjustments	529,696	107,969	326,875
Net cash provided by operating activities	400,645	514,895	694,345
Property, plant and equipment:
Capital expenditures	(121,101)	(305,858)	(233,480)
Decrease in accounts payable and accrued liabilities	(8,773)	(81)	(1,051)
Proceeds from sale of property, plant and equipment	3,762	1,266	2,409
Contributions to equity method investments			(15,600)
Distributions received from investments in excess of cumulative earnings	988	2,501	2,473
Payments for acquisitions of businesses, net of cash acquired		(320,232)
Cash received from redemption of equity securities		134,288
Net cash used in investing activities	(125,124)	(488,116)	(245,249)
CASH FLOWS FROM FINANCING ACTIVITIES:
Borrowings under securitization facility	46,100	184,500	304,600
Payments under securitization facility	(64,000)	(202,700)	(285,000)
Proceeds from equipment financings	14,705	63,086
Payments on equipment financings	(14,805)	(2,607)
Borrowings under revolving credit facilities	70,000	400,000	245,000
Payments under revolving credit facilities	(237,500)	(320,000)	(100,000)
Payments on finance lease obligations	(8,368)	(46,725)	(29,353)
Payment of debt issuance costs	(6,280)
Payments for purchases of units under unit repurchase program		(22,892)	(70,604)
Payments for taxes related to net settlement of issuance of units in deferred compensation plans	(1,310)	(7,817)	(2,081)
cs under deferred compensation plan	(2,490)
Cash contributions by General Partner			41
Cash contribution by affiliated entity			2,142
Cash obtained in Simplification Transactions			1,139
Distributions paid to Partners	(51,753)	(278,425)	(275,902)
Other	(728)	(867)	(1,684)
Net cash used in financing activities	(256,429)	(234,447)	(211,702)
NET CHANGE IN CASH AND CASH EQUIVALENTS	19,092	(207,668)	237,394
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	36,482	244,150	6,756
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ 55,574	$ 36,482	$ 244,150

CONSOLIDATED STATEMENT OF PARTN

CONSOLIDATED STATEMENT OF PARTNERS' CAPITAL - USD ($) $ in Thousands	Limited Partners' Capital	General Partner's Capital (Deficit)	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest	Total
Balance at beginning of period at Dec. 31, 2017	$ 1,183,219	$ 14,859	$ (51,940)	$ 5,348	$ 1,151,486
Balance at beginning of period (in units) at Dec. 31, 2017	130,704,217
Comprehensive income:
Net income (loss)	$ 365,044	1,560		866	367,470
Actuarially determined long-term liability adjustments			5,069		5,069
COMPREHENSIVE INCOME (LOSS)					372,539
Settlement of deferred compensation plans	$ (2,745)				(2,745)
Settlement of deferred compensation plans (in units)	199,039
Issuance of units to SGP in Exchange Transaction (in units)	20,960
Issuance of units to Owners of SGP in Simplification Transactions	$ 14,742	(15,106)			(364)
Issuance of units to Owners of SGP in Simplification Transactions (in units)	1,322,388
Simplification Transactions fees	$ (96)				(96)
Contribution of units and cash by affiliated entity	$ 2,142				2,142
Contribution of units by affiliated entity (in units)	(467,018)
Purchase of units under unit repurchase program	$ (70,604)				(70,604)
Purchase of units under unit repurchase program (in units)	(3,684,075)
Common unit-based compensation	$ 12,114				12,114
Distributions on deferred common unit-based compensation	(3,855)				(3,855)
General Partners contributions		41			41
Distributions from consolidated company to affiliate noncontrolling interest				(924)	(924)
Distributions to Partners	(270,693)	$ (1,354)			(272,047)
Balance at end of period at Dec. 31, 2018	$ 1,229,268		(46,871)	5,290	1,187,687
Balance at end of period (in units) at Dec. 31, 2018	128,095,511
Comprehensive income:
Net income (loss)	$ 399,414			7,512	406,926
Actuarially determined long-term liability adjustments			(31,122)		(31,122)
COMPREHENSIVE INCOME (LOSS)					375,804
Settlement of deferred compensation plans	$ (7,817)				(7,817)
Settlement of deferred compensation plans (in units)	596,650
Purchase of units under unit repurchase program	$ (22,892)				(22,892)
Purchase of units under unit repurchase program (in units)	(1,776,564)
Common unit-based compensation	$ 11,934				11,934
Distributions on deferred common unit-based compensation	(3,670)				(3,670)
Distributions from consolidated company to affiliate noncontrolling interest				(867)	(867)
Distributions to Partners	(274,755)				(274,755)
Balance at end of period at Dec. 31, 2019	$ 1,331,482		(77,993)	11,935	$ 1,265,424
Balance at end of period (in units) at Dec. 31, 2019	126,915,597				126,915,597
Comprehensive income:
Net income (loss)	$ (129,220)			169	$ (129,051)
Actuarially determined long-term liability adjustments			(9,681)		(9,681)
COMPREHENSIVE INCOME (LOSS)					(138,732)
Settlement of deferred compensation plans	$ (3,800)				(3,800)
Settlement of deferred compensation plans (in units)	279,622
Common unit-based compensation	$ 3,345				3,345
Distributions on deferred common unit-based compensation	(986)				(986)
Distributions from consolidated company to affiliate noncontrolling interest				(728)	(728)
Distributions to Partners	(50,767)				(50,767)
Other	(1,489)				(1,489)
Balance at end of period at Dec. 31, 2020	$ 1,148,565		$ (87,674)	$ 11,376	$ 1,072,267
Balance at end of period (in units) at Dec. 31, 2020	127,195,219				127,195,219

ORGANIZATION AND PRESENTATION

ORGANIZATION AND PRESENTATION	12 Months Ended
ORGANIZATION AND PRESENTATION	Dec. 31, 2020
ORGANIZATION AND PRESENTATION
ORGANIZATION AND PRESENTATION	1. ORGANIZATION AND PRESENTATION Significant Relationships Referenced in Notes to Consolidated Financial Statements ● References to "we," "us," "our" or "ARLP Partnership" mean the business and operations of Alliance Resource Partners, L.P., the parent company, as well as its consolidated subsidiaries. ● References to "ARLP" mean Alliance Resource Partners, L.P., individually as the parent company, and not on a consolidated basis. ● References to "MGP" mean Alliance Resource Management GP, LLC, ARLP's general partner. ● References to "Mr. Craft" mean Joseph W. Craft III, the Chairman, President and Chief Executive Officer of MGP. ● References to "SGP" mean Alliance Resource GP, LLC. SGP is indirectly wholly owned by Mr. Craft and Kathleen S. Craft, who are collectively referred to in such capacity as the "Owners of SGP." The Owners of SGP held approximately 34.48% of the outstanding AHGP common units prior to the Simplification Transactions discussed below. SGP was dissolved on December 30, 2020 and is in the process of winding up its affairs. ● References to "Intermediate Partnership" mean Alliance Resource Operating Partners, L.P., the intermediate partnership of Alliance Resource Partners, L.P. ● References to "Alliance Coal" mean Alliance Coal, LLC, the holding company for the coal mining operations of Alliance Resource Operating Partners, L.P. ● References to "Alliance Minerals" mean Alliance Minerals, LLC, the holding company for the oil and gas minerals interests of Alliance Resource Partners, L.P. ● References to "AHGP" mean Alliance Holdings GP, L.P., individually and not on a consolidated basis as the parent company of MGP prior to the Simplification Transactions discussed below and as a wholly owned subsidiary of ARLP subsequent to the Simplification Transactions. Organization ARLP is a Delaware limited partnership listed on the NASDAQ Global Select Market under the ticker symbol "ARLP." ARLP was formed in May 1999 and completed its initial public offering on August 19, 1999 when it acquired substantially all of the coal production and marketing assets of Alliance Resource Holdings, Inc., a Delaware corporation ("ARH"), and its subsidiaries. We are managed by our general partner, MGP, a Delaware limited liability company which holds a non-economic general partner interest in ARLP. Prior to the Simplification Transactions, MGP was a wholly owned indirect subsidiary of AHGP. Alliance GP, LLC ("AGP"), which is indirectly wholly owned by Mr. Craft, was the general partner of AHGP prior to the Simplification Transactions and became the direct owner of MGP as a result of the transactions. See discussions under Partnership Simplification Partnership Simplification On February 22, 2018, the board of directors ("Board of Directors") of MGP and the board of directors of AGP approved a simplification agreement (the "Simplification Agreement"), pursuant to which, among other things, through a series of transactions (the "Simplification Transactions"): i. AHGP would become a wholly owned subsidiary of ARLP, ii. all of the issued and outstanding AHGP common units would be canceled and converted into the right to receive the ARLP common units held by AHGP and its subsidiaries, iii. in exchange for a number of ARLP common units calculated pursuant to the Simplification Agreement, MGP's 1.0001% general partner interest in our Intermediate Partnership and MGP's 0.001% managing member interest in our subsidiary, Alliance Coal, would be contributed to us, and iv. MGP would remain ARLP's general partner and would be a wholly owned subsidiary of AGP, and thus no control, management, or governance changes with respect to our business would occur. The Simplification Agreement and the transactions contemplated thereby were approved by the written consent of approximately 68% of the holders of AHGP common units outstanding as of April 25, 2018, the record date for the consent solicitation. On May 31, 2018, ARLP, AHGP and the other parties to the Simplification Agreement completed the transactions contemplated by the Simplification Agreement. As part of the Simplification Transactions, (i) each AHGP common unit that was issued and outstanding at the effective time of the Simplification Transactions was canceled and converted into the right to receive a portion of the ARLP common units held by AHGP and its subsidiaries, and (ii) SGP became the sole limited partner in AHGP. Each outstanding AHGP common unit, other than certain AHGP common units held by the Owners of SGP, converted into the right to receive approximately 1.4782 ARLP common units held by AHGP and its subsidiaries. The remaining AHGP common units held by the Owners of SGP were canceled and converted into the right to receive 29,188,997 ARLP common units which equaled (i) the product of the number of certain AHGP common units held by the Owners of SGP multiplied by 1.4782, minus (ii) 1,322,388 ARLP common units. In addition, ARLP issued 1,322,388 ARLP common units to the Owners of SGP in exchange for causing SGP to contribute to ARLP its remaining limited partner interest in AHGP, which included AHGP's indirect ownership of a 1.0001% general partner interest in the Intermediate Partnership and a 0.001% managing member interest in Alliance Coal, resulting in an overall exchange ratio to the Owners of SGP equal to that of the other AHGP unitholders. Upon the issuance of ARLP common units to the Owners of SGP in exchange for the limited partner interest in AHGP, ARLP became a) the sole limited partner of AHGP and b) through AHGP, the indirect owner of a 1.0001% general partner interest in the Intermediate Partnership and a 0.001% managing member interest in Alliance Coal. AllDale I & II Acquisition On January 3, 2019 (the "AllDale Acquisition Date"), we acquired all of the limited partner interests not owned by Cavalier Minerals JV, LLC ("Cavalier Minerals") in AllDale Minerals LP ("AllDale I") and AllDale Minerals II, LP ("AllDale II", and collectively with AllDale I, "AllDale I & II") and the general partner interests in AllDale I & II (the "AllDale Acquisition"). As a result of the AllDale Acquisition and our previous investments held through Cavalier Minerals, we acquired control of approximately 43,000 net royalty acres in premier oil & gas resource plays. The AllDale Acquisition provides us with diversified exposure to industry leading operators and is consistent with our general business strategy to grow our Minerals segment. See Note 3 – Acquisitions for more information. Wing Acquisition On August 2, 2019, our subsidiary AR Midland, LP ("AR Midland") acquired from Wing Resources LLC and Wing Resources II LLC (collectively, "Wing") approximately 9,000 net royalty acres in the Midland Basin, with exposure to more than 400,000 gross acres (the "Wing Acquisition"). The Wing Acquisition enhances our ownership position in the Permian Basin, expands our exposure to industry leading operators and furthers our business strategy to grow our Minerals segment. Following the Wing Acquisition, we hold approximately 55,700 net royalty acres in premier oil & gas basins including our investment in AllDale Minerals III, LP ("AllDale III"). See Note 3 – Acquisitions for more information. Presentation The consolidated financial statements include the accounts and operations of the ARLP Partnership and present our financial position as of December 31, 2020 and 2019, and results of our operations, comprehensive income, cash flows and changes in partners' capital for each of the three years in the period ended December 31, 2020. All of our intercompany transactions and accounts have been eliminated.

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	Dec. 31, 2020
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Consolidation Variable Interest Entity ("VIE") To determine a VIE's primary beneficiary, we perform a qualitative assessment to determine which party, if any, has the power to direct activities of the VIE and the obligation to absorb losses and/or receive its benefits. This assessment involves identifying the activities that most significantly impact the VIE's economic performance and determine whether it, or another party, has the power to direct those activities. When evaluating whether we are the primary beneficiary of a VIE, we perform a qualitative analysis that considers the design of the VIE, the nature of our involvement and the variable interests held by other parties. See Note 12 – Variable Interest Entities for further information. Estimates ● Impairment assessments of investments, property, plant and equipment, and goodwill; ● Asset retirement obligations; ● Pension valuation variables; ● Workers' compensation and pneumoconiosis valuation variables; ● Acquisition related purchase price allocations; ● Life of mine assumptions; ● Oil & gas reserve quantities and carrying amounts; and ● Determination of oil & gas revenue accruals These significant estimates and assumptions are discussed throughout these notes to the consolidated financial statements. Fair Value Measurements We use the following fair value hierarchy, which prioritizes the inputs to valuation techniques used to measure fair value into three broad levels: ● Level 1 – Quoted prices for identical assets and liabilities in active markets that we have the ability to access at the measurement date. ● Level 2 – Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model derived valuations whose inputs are observable or whose significant value drivers are observable. ● Level 3 – Unobservable inputs for the asset or liability including situations where there is little, if any, market activity for the asset or liability. The fair value hierarchy gives the highest priority to quoted prices in active markets (Level 1) and the lowest priority to unobservable data (Level 3). In some cases, the inputs used to measure fair value might fall into different levels of the fair value hierarchy. The lowest level input that is significant to a fair value measurement determines the applicable level in the fair value hierarchy. Assessing the significance of a particular input to the fair value measurement requires judgment, considering factors specific to the asset or liability. Significant fair value measurements are used in our significant estimates and are discussed throughout these notes. Cash and Cash Equivalents Cash Management Inventories Business Combinations Goodwill Property, Plant and Equipment Mine Development Costs Leases lease obligations. Some of our arrangements within these agreements have both lease and non-lease components, which are generally accounted for separately. We have elected a practical expedient to account for lease and non-lease components as a single lease component for leases of buildings and office equipment. Our leases have approximate lease terms of one year to 20 years, some of which include automatic renewals up to ten years which are likely to be exercised, and some of which include options to terminate the lease within one year. We also hold numerous mineral reserve leases with both related parties as well as third parties, none of which are accounted for as an operating lease or as a finance lease. We review each agreement to determine if an arrangement within the agreement contains a lease at the inception of an arrangement. Once an arrangement is determined to contain either an operating or finance lease with a term greater than 12 months, we recognize a lease liability for the obligation to make lease payments and a right-of-use asset for the right to use the underlying asset for the lease term based on the present value of lease payments over the lease term. The lease term includes all noncancelable periods defined in the lease as well as periods covered by options to extend the lease that we are reasonably certain to exercise. As an implicit borrowing rate cannot be determined under most of our leases, we use our incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. Expenses related to leases determined to be operating leases will be recognized on a straight-line basis over the lease term including any reasonably assured renewal periods, while those determined to be finance leases will be recognized following a front-loaded expense profile in which interest and amortization are presented separately in the income statement. The determination of whether a lease is accounted for as a finance lease or an operating lease requires management to make estimates primarily about the fair value of the asset and its estimated economic useful life. Long-Lived Asset Impairment Oil & Gas Reserve Quantities and Carrying Amounts Mineral interests in oil & gas properties are grouped using a reasonable aggregation of properties with a common geological structural feature or stratigraphic condition, which we may also refer to as a depletable group. Mineral interests in proved oil & gas properties are depleted based on the units-of-production method. Proved reserves are quantities of oil & gas that can be estimated with reasonable certainty to be recoverable in the future from a given date forward, from known reservoirs, under existing economic conditions, operating methods, and government regulations. Proved developed resources are the quantities expected to be recovered through our operators' existing wells with existing equipment, infrastructure and operating methods. We evaluate impairment of our mineral interests in proved properties whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. This evaluation is performed on a depletable group basis. We compare the undiscounted projected future cash flows expected in connection with a depletable group to its unamortized carrying amount to determine recoverability. When the carrying amount of a depletable group exceeds its estimated undiscounted future cash flows, the carrying amount is written down to its fair value, which is measured as the present value of the projected future cash flows of such properties. The factors used to determine fair value include estimates of proved reserves, future commodity prices, timing of future production, future expenditures, and a risk-adjusted discount rate. Our mineral interests in unproved properties are also assessed for impairment periodically on a depletable group basis when facts and circumstances indicate that the carrying value may not be recoverable. Impairment of individual unproved properties whose acquisition costs are relatively significant are assessed on a property-by-property basis, and an impairment loss is recognized to the extent the carrying value exceeds the estimated recoverable value for the property. Impairment of unproved properties whose acquisition costs are not individually significant are assessed on a group basis. Any amount of loss to be recognized and the amount of a valuation allowance needed to provide for impairment of those properties is determined by amortizing those properties in the aggregate on the basis of historical experience and other relevant information, such as the relative proportion of such properties on which proved reserves have been found in the past. The carrying value of unproved properties, including unleased mineral rights, are determined based on management's assessment of fair value using factors similar to those previously noted for proved properties, as well as geographic and geologic data. Upon the sale of a complete depletable group, the book value thereof, less proceeds or salvage value, are charged to income. Upon the sale or retirement of an aggregation of interests which make up less than a complete depletable group, the proceeds are credited to accumulated depreciation, depletion and amortization, unless doing so would significantly alter the depreciation, depletion and amortization rate of the depletable group, in which case a gain or loss would be recorded. Intangibles Prepaid expenses and other assets Other long-term assets December 31, 2020 December 31, 2019 Accumulated Intangibles, Accumulated Intangibles, Original Cost Amortization Net Original Cost Amortization Net (in thousands) Non-compete agreements $ — $ — $ — $ 9,803 $ (9,440) $ 363 Customer contracts and other 10,623 (5,744) 4,879 32,371 (24,258) 8,113 Mining permits 1,500 (373) 1,127 1,500 (307) 1,193 Total $ 12,123 $ (6,117) $ 6,006 $ 43,674 $ (34,005) $ 9,669 Amortization expense attributable to intangible assets is estimated as follows: Year Ended December 31, (in thousands) 2021 $ 2,831 2022 1,600 2023 647 2024 66 2025 66 Thereafter 795 Investments Our investments and ownership interests in entities in which we do not have a controlling financial interest are accounted for under the equity method of accounting if we have the ability to exercise significant influence over the entity. Investments accounted for under the equity method are initially recorded at cost, and the difference between the basis of our investment and the underlying equity in the net assets of the joint venture at the investment date, if any, is amortized over the lives of the related assets that gave rise to the difference. As of December 31, 2020 and 2019, we held an equity method investment in AllDale III through our subsidiary, Alliance Minerals. Prior to the AllDale Acquisition, our equity method investments also included AllDale I & II, both held through Cavalier Minerals. AllDale III and AllDale I & II are collectively referred to as the "AllDale Partnerships." See Note 13 – Investments for further discussion of our equity method investment in AllDale III and Note 3 – Acquisitions for discussion of the AllDale Acquisition. We review our investments for impairment whenever events or changes in circumstances indicate a loss in the value of the investment may be other-than-temporary. Advance Royalties Advance royalties December 31, 2020 2019 (in thousands) Advance royalties, affiliates (see Note 21 – Related-Party Transactions) $ 48,389 $ 41,216 Advance royalties, third-parties 12,570 12,685 Total advance royalties $ 60,959 $ 53,901 Asset Retirement Obligations Pension Benefits The discount rate is determined for our pension benefit plan based on an approach specific to our plan. The year end discount rate is determined considering a yield curve comprised of high-quality corporate bonds and the timing of the expected benefit cash flows. The expected long-term rate of return on plan assets is determined based on broad equity and bond indices, the investment goals and objectives, the target investment allocation and on the average annual total return for each asset class. Unrecognized actuarial gains and losses and unrecognized prior service costs and credits are deferred and recorded in accumulated other comprehensive loss until amortized as a component of net periodic benefit cost. Unrecognized actuarial gains and losses in excess of 10% of the greater of the benefit obligation or the market-related value of plan assets are amortized over the participants' average remaining future years of service. Workers Compensation and Pneumoconiosis (Black Lung) Benefits We provide income replacement and medical treatment for work-related traumatic injury claims as required by applicable state laws. Workers' compensation laws also compensate survivors of workers who suffer employment related deaths. Our liability for traumatic injury claims is the estimated present value of current workers' compensation benefits, based on our actuarial estimates. Our actuarial calculations are based on a blend of actuarial projection methods and numerous assumptions including claim development patterns, mortality, medical costs and interest rates. Our pneumoconiosis benefits liability is calculated using the service cost method based on the actuarial present value of the estimated pneumoconiosis obligation. Our actuarial calculations are based on numerous assumptions including claim development patterns, medical costs and mortality. Actuarial gains or losses are amortized over the remaining service period of active miners. See Note 20 – Accrued Workers' Compensation and Pneumoconiosis Benefits for more information on Workers' Compensation and Pneumoconiosis Benefits. Coal Revenue Recognition The estimated transaction price from each of our contracts is based on the total amount of consideration we expect to be entitled to under the contract. Included in the transaction price for certain coal supply contracts is the impact of variable consideration, including quality price adjustments, handling services, government imposition claims, per ton price fluctuations based on certain coal sales price indices and anticipated payments in lieu of shipments. We have constrained the expected value of variable consideration in our estimation of transaction price and only included this consideration to the extent that it is probable that a significant revenue reversal will not occur. The estimated transaction price for each contract is allocated to our performance obligations based on relative standalone selling prices determined at contract inception. Variable consideration is allocated to a specific part of the contract in many instances, such as if the variable consideration is based on production activities for coal delivered during a certain period or the outcome of a customer's ability to accept coal shipments over a certain period. Contract assets are recorded as trade receivables and reported separately in our consolidated balance sheet from other contract assets as title passes to the customer and our right to consideration becomes unconditional. Payments for coal shipments are typically due within two four weeks Oil & Gas Revenue Recognition among other factors, whether a well connects to a gathering or transmission line, quality and heat content of the product, and prevailing supply and demand conditions. We also periodically earn revenue from lease bonuses. We recognize lease bonus revenue when we execute a lease of our mineral interests to exploration and production companies. A lease agreement represents our contract with an operator, which is generally an exploration and production company. The contract will a) generally transfer the rights to any oil or gas discovered, b) grant us a right to a specified royalty interest from the operator, and c) require the operator to commence drilling and complete operations within a specified time period. Control of the minerals transfers to the operator when the lease agreement is executed. At the time we execute the lease agreement, we expect to receive the lease bonus payment within a reasonable time, though in no case more than one year, such that we do not adjust the expected amount of consideration for the effects of any significant financing component. As a non-operator, we have limited visibility into the timing of when new wells start producing. In addition, production statements may not be received for 30 to 90 days or more after the date production is delivered. As a result, we are required to estimate the amount of production delivered to the purchaser and the price that will be received for the sale of the product. The expected sales volumes and prices from our properties are estimated and recorded within the Trade receivables Common Unit-Based Compensation We utilize the Supplemental Executive Retirement Plan ("SERP") to provide deferred compensation benefits for certain officers and key employees. All allocations made to participants under the SERP are made in the form of "phantom" ARLP units and SERP distributions will be settled in the form of ARLP common units. The SERP is administered by the Compensation Committee. Our directors participate in the MGP Amended and Restated Deferred Compensation Plan for Directors ("Directors' Deferred Compensation Plan"). Pursuant to the Directors' Deferred Compensation Plan, for amounts deferred either automatically or at the election of the director, a notional account is established and credited with notional common units of ARLP, described in the Directors' Deferred Compensation Plan as "phantom" units. Distributions from the Directors' Deferred Compensation Plan will be settled in the form of ARLP common units. For both the SERP and Directors' Deferred Compensation Plan, when quarterly cash distributions are made with respect to ARLP common units, an amount equal to such quarterly distribution is credited to each participant's notional account as additional phantom units. All grants of phantom units under the SERP and Directors' Deferred Compensation Plan vest immediately. The fair value of restricted common unit grants under the LTIP, SERP and the Directors' Deferred Compensation Plan are determined on the grant date of the award and recognized as compensation expense on a pro rata basis for LTIP and SERP awards, as appropriate, over the requisite service period. Compensation expense is fully recognized on the grant date for quarterly distributions credited to SERP accounts and Directors' Deferred Compensation Plan awards. The corresponding liability is classified as equity and included in limited partners' capital in the consolidated financial statements (See Note 17 – Compensation Plans). Income Taxes New Accounting Standards Issued and Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments–Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"). ASU 2016-13 changes the impairment model for most financial assets and certain other instruments to require the use of a new forward-looking "expected loss" model that generally will result in earlier recognition of allowances for losses. The new standard provides for the use of a modified retrospective transition method that allows for a cumulative-effect adjustment to retained earnings upon adoption. The new standard also requires disclosure of significantly more information related to these items. We adopted ASU 2016-13 on January 1, 2020. Because of the credit profile of our customers, the fact that we do not have a history of credit losses on our financial instruments and the absences of any material expected losses, the adoption of ASU 2016-13 did not have any material impact on our consolidated financial statements.

ACQUISITIONS

ACQUISITIONS	12 Months Ended
ACQUISITIONS	Dec. 31, 2020
ACQUISITION
ACQUISITION	3. ACQUISITION S AllDale I & II On the AllDale Acquisition Date, we acquired all of the limited partner interests not owned by Cavalier Minerals in AllDale I & II and the general partner interests in AllDale I & II for $176.2 million, which was funded with cash on hand and borrowings under the Revolving Credit Facility. As a result of the AllDale Acquisition and our previous investments held through Cavalier Minerals, we acquired control of approximately 43,000 net royalty acres strategically positioned primarily in the core of the Permian (Delaware and Midland), Anadarko (SCOOP/STACK) and Williston (Bakken) Basins. The AllDale Acquisition provides us with diversified exposure to industry leading operators and is consistent with our general business strategy to grow our Minerals segment. Because the underlying mineral interests held by AllDale I & II include royalty interests in both producing properties and unproved properties, we have determined that the AllDale Acquisition should be accounted for as a business combination and the underlying assets and liabilities of AllDale I & II should be recorded at their AllDale Acquisition Date fair value on our consolidated balance sheet. The final total fair value of the cash paid in the AllDale Acquisition and our previous investments were as follows: As of January 3, 2019 (in thousands) Cash $ 176,205 Previously held investments 307,322 Total $ 483,527 Prior to the AllDale Acquisition Date, we accounted for our investments in AllDale I & II, held through Cavalier Minerals, as equity method investments. The combined fair value of our equity method investments on the AllDale Acquisition Date was $307.3 million. We re-measured our equity method investments, which had an aggregate carrying value of $130.3 million immediately prior to the AllDale Acquisition. The re-measurement resulted in a gain of $177.0 million which is recorded in the Acquisition gain line item in our consolidated statements of income. The following table summarizes the final fair value allocation of assets acquired and liabilities assumed as of the AllDale Acquisition Date: (in thousands) Cash and cash equivalents $ 900 Mineral interests in proved properties 184,032 Mineral interests in unproved properties 291,190 Receivables 9,326 Accounts payable (1,921) Net assets acquired $ 483,527 Our previous equity method investments in AllDale I & II were held through Cavalier Minerals. Bluegrass Minerals Management, LLC ("Bluegrass Minerals") continues to hold a 4% membership interest (the "Bluegrass Interest") as well as a profits interest in Cavalier Minerals as it did before the AllDale Acquisition. This Bluegrass Interest represents an indirect noncontrolling interest in AllDale I & II. The AllDale Acquisition Date fair value of the Bluegrass Interest was $12.3 million. The fair value of our previous equity method investments, the mineral interests and the Bluegrass Interest were determined using an income approach primarily comprised of discounted cash flow models. The assumptions used in the discounted cash flow models include estimated production, projected cash flows, forward oil & gas prices and a risk adjusted discount rate. Certain assumptions used are not observable in active markets, therefore the fair value measurements represent Level 3 fair value measurements. AllDale I & II's carrying value of the receivables and accounts payable represent their fair value given their short-term nature. The amounts of revenue and earnings, exclusive of the acquisition gain, of AllDale I & II included in our consolidated statements of income from the AllDale Acquisition Date through December 31, 2019 are as follows: Year Ended December 31, 2019 (in thousands) Revenue $ 48,411 Net income 18,543 The following represents our actual and pro forma consolidated revenues and net income for the year ended December 31, 2018. Pro forma revenues and net income assumes AllDale I & II had been included in our consolidated results since January 1, 2018. These amounts have been calculated after applying our accounting policies. Pro forma information is not necessary for the year ended December 31, 2019 as the AllDale Acquisition occurred at the beginning of 2019. Additionally, our pro forma results have been adjusted to remove the effect of our past equity method investments in AllDale I & II. Year Ended December 31, 2018 (in thousands) Total revenues As reported $ 2,002,857 Pro forma 2,042,545 Net income As reported $ 367,470 Pro forma 358,741 Wing On August 2, 2019 (the "Wing Acquisition Date"), our subsidiary, AR Midland acquired from Wing approximately 9,000 net royalty acres in the Midland Basin, with exposure to more than 400,000 gross acres, for a cash purchase price of $144.9 million. The purchase price was funded with cash on hand and borrowings under our Revolving Credit Facility discussed in Note 8 – Long-Term Debt. The Wing Acquisition enhances our ownership position in the Permian Basin, expands our exposure to industry leading operators and furthers our business strategy to grow our Minerals segment. Concurrent with the Wing Acquisition, JC Resources LP, an entity owned by Mr. Craft, acquired from Wing, in a separate transaction, mineral interests that we elected not to acquire. Because the mineral interests acquired in the Wing Acquisition include royalty interests in both producing properties and unproved properties, we have determined that the acquisition should be accounted for as a business combination and the underlying assets should be recorded at fair value as of the Wing Acquisition Date on our consolidated balance sheet. During the year ended December 31, 2020, we recorded adjustments to our mineral interests in proved and unproved properties after receiving additional information regarding proved and unproved reserve quantities, production and projections as of the Wing Acquisition Date. In addition, we increased our receivables by $0.3 million as a result of information received from operators concerning royalty payments owed to us from production that occurred prior to the Wing Acquisition Date. The following table summarizes our final fair value allocation of assets acquired as of the Wing Acquisition Date incorporating measurement period adjustments made to the allocation: As Previously Reported Adjustments Final (in thousands) Mineral interests in proved properties $ 58,084 16,987 $ 75,071 Mineral interests in unproved properties 84,976 (17,275) 67,701 Receivables 1,867 288 2,155 Net assets acquired $ 144,927 $ 144,927 The fair value of the mineral interests was determined using a weighting of both income and market approaches. Our income approach primarily comprised a discounted cash flow model. The assumptions used in the discounted cash flow model included estimated production, projected cash flows, forward oil & gas prices and a weighted average cost of capital. Our market approach consisted of the observation of recent acquisitions in the Permian Basin to determine a market price for similar mineral interests. Certain assumptions used in our valuation are not observable in active markets; therefore, the fair value measurements represent Level 3 fair value measurements. The carrying value of the receivables represents the fair value given the short-term nature of the receivables. The amounts of revenue and earnings from the mineral interests acquired in the Wing Acquisition included in our consolidated statements of income from the Wing Acquisition Date through December 31, 2019 are as follows: Year Ended December 31, 2019 (in thousands) Revenue $ 4,625 Net income 1,291 The following represents our actual and pro forma consolidated revenues and net income for the years ended December 31, 2019 and 2018. Pro forma revenues and net income assumes the mineral interests acquired in the Wing Acquisition had been included in our consolidated results since January 1, 2018. These pro forma amounts have been calculated after applying our accounting policies. Year Ended December 31, 2019 2018 (in thousands) Total revenues As reported $ 1,961,720 $ 2,002,857 Pro forma 1,966,291 2,008,559 Net income As reported $ 406,926 $ 367,470 Pro forma 411,217 372,810

LONG-LIVED ASSET IMPAIRMENTS

LONG-LIVED ASSET IMPAIRMENTS	12 Months Ended
LONG-LIVED ASSET IMPAIRMENTS	Dec. 31, 2020
LONG-LIVED ASSET IMPAIRMENTS
LONG-LIVED ASSET IMPAIRMENTS	4. LONG-LIVED ASSET IMPAIRMENT S During the year ended December 31, 2020, we recorded $25.0 million of non-cash asset impairment charges in our Illinois Basin segment due to sealing our idled Gibson North mine, resulting in its permanent closure, and a decrease in the fair value of certain mining equipment at our idled operations and greenfield coal reserves as a result of weakened coal market conditions. During the year ended December 31, 2019, we recorded an asset impairment charge of $15.2 million in our Illinois Basin segment due to the cessation of coal production at our Dotiki mine, effective August 16, 2019, in an effort to focus on maximizing production at our lower-cost mines in the segment. We adjusted the carrying value of Dotiki's assets from $35.9 million to its fair value of $25.8 million and accrued $5.1 million with respect to scheduled payments to WKY CoalPlay for leased coal reserves from which we may not receive future economic benefit. See Note 12 – Variable Interest Entities for more information about WKY CoalPlay. During the year ended December 31, 2018, due to the reduction of Dotiki’s economic mine life, we recorded a $34.3 million impairment charge when we adjusted the carrying value of Dotiki's assets from $85.3 million to its fair value of $51.0 million. We also had a decrease in the fair value of an option entitling us to lease certain coal reserves, which resulted in an impairment charge of $6.2 million. Both of these impairment charges were incurred in our Illinois Basin segment. The fair values of the impaired assets were determined using a combination of market and income approaches, both of which represent Level 3 fair value measurements under the fair value hierarchy. The fair value analysis used assumptions of marketability of certain assets as well as discounted cash flows over the remaining life of the assets. With the uncertainty related to energy demand as a result of weak electricity demand and an oversupply and lack of storage for oil and natural gas during the quarter ended March 31, 2020 (the "First Quarter"), both due in part to the COVID-19 pandemic and other market and production factors impacting both our coal mining operations and our mineral interests activities, we performed recoverability tests during the First Quarter using undiscounted cash flows based on our estimate of sales volume and prices, operating margins and capital expenditures from information available to us and determined we would be able to recover the costs of our assets, excluding the impaired assets discussed above. Amid cost reduction efforts, increased customer commitments for coal, a modest recovery in commodity futures prices and increased clarity into production levels by operators of our oil & gas mineral interests during the year, we determined impairment of our long-lived assets subsequent to the First Quarter was not necessary. The cash flow estimates used in our impairment assessments, by their very nature, are dependent on conditions that could materially change in future periods based on new information. If in future periods changes to these estimates were to materially reduce our expected cash flows, additional impairments could be necessary. See Note 2 – Summary of Significant Accounting Policies – Long-Lived Asset Impairment for more information on our accounting policy for asset impairments.

GOODWILL IMPAIRMENT

GOODWILL IMPAIRMENT	12 Months Ended
GOODWILL IMPAIRMENT	Dec. 31, 2020
GOODWILL IMPAIRMENT
GOODWILL IMPAIRMENT	5. GOODWILL IMPAIRMENT At December 31, 2019, our consolidated balance sheet included $136.4 million of goodwill, of which $132.0 million was associated with the reporting unit representing our Hamilton County Coal, LLC ("Hamilton") mine, which is included in our Illinois Basin segment. The goodwill associated with our Hamilton mine was recorded in conjunction with our acquisition of the Hamilton mine on July 31, 2015. During the First Quarter, we assessed certain events and changes in circumstances, including a) adverse industry and market developments, including the impact of the COVID-19 pandemic, b) our response to these developments, including temporarily ceasing production at several mines, including Hamilton and c) our actual performance during the First Quarter. After consideration of these events and changes in circumstances, we performed an interim test of the goodwill associated with the Hamilton reporting unit comparing Hamilton's carrying amount to its fair value. We estimated the fair value of the Hamilton reporting unit using an income approach utilizing a discounted cash flow model. The assumptions used in the discounted cash flow model included estimated production, forward coal prices, operating expenses, capital expenditures and a weighted average cost of capital. Our forecasts of future cash flows considered market conditions at the time of the assessment and our estimate of the mine's performance in future years based on the information available to us. Key assumptions used in our valuation are not observable in active markets; therefore, the fair value measurements represent Level 3 fair value measurements. The fair value of the Hamilton reporting unit was determined to be below its carrying amount (including goodwill) by more than the recorded balance of goodwill associated with the reporting unit. Accordingly, we recognized an impairment charge of $132.0 million consisting of the total carrying amount of goodwill allocated to the Hamilton reporting unit. This impairment charge reduced our consolidated goodwill balance to $4.4 million. During the First Quarter and as part of our annual impairment evaluation on November 30, 2020, we also performed tests on ARLP's remaining goodwill balances not associated with Hamilton and concluded no impairment was necessary for our other reporting units.

INVENTORIES

INVENTORIES	12 Months Ended
INVENTORIES	Dec. 31, 2020
INVENTORIES
INVENTORIES	6. INVENTORIES Inventories consist of the following : December 31, 2020 2019 (in thousands) Coal $ 19,756 $ 63,645 Supplies (net of reserve for obsolescence of $5,547 and $5,555, respectively) 36,651 37,660 Total inventories, net $ 56,407 $ 101,305 For the year ended December 31, 2020, we recorded lower of cost or net realizable value adjustments of $3.2 million to our coal inventories as a result of lower coal sale prices and higher cost per ton due to the impact of lower production on our fixed costs per ton in addition to the impact of challenging market conditions on our production levels. The lower of cost or net realizable value adjustments reflect the impacts of the challenging market conditions and were primarily attributable to the Mettiki and Hamilton mining complexes. See Note 2 – Summary of Significant Accounting Policies for more information on our accounting policy for inventories.

PROPERTY, PLANT AND EQUIPMENT

PROPERTY, PLANT AND EQUIPMENT	12 Months Ended
PROPERTY, PLANT AND EQUIPMENT	Dec. 31, 2020
PROPERTY, PLANT AND EQUIPMENT
PROPERTY, PLANT AND EQUIPMENT	7. PROPERTY, PLANT AND EQUIPMENT Property, plant and equipment consist of the following: December 31, 2020 2019 (in thousands) Mining equipment and processing facilities $ 1,896,324 $ 1,937,642 Land and coal mineral rights 454,310 453,237 Oil & gas mineral interests (1) 616,904 618,282 Buildings, office equipment and improvements 279,938 304,111 Construction and mine development in progress 25,799 86,876 Mine development costs 280,815 283,860 Property, plant and equipment, at cost 3,554,090 3,684,008 Less accumulated depreciation, depletion and amortization (1,753,845) (1,675,022) Total property, plant and equipment, net $ 1,800,245 $ 2,008,986 (1) Oil & gas mineral interests acquired in the AllDale and Wing Acquisitions. See Note 3 – Acquisitions for more information. At December 31, 2020 and 2019, land and coal mineral rights above include $37.5 million and $40.1 million, respectively, of carrying value associated with coal reserves attributable to properties where we or a third party to which we lease reserves are not currently engaged in mining operations or leasing to third parties, and therefore, the coal reserves are not currently being depleted. We believe that the carrying value of these coal reserves will be recovered. At December 31, 2020 and 2019, our oil & gas mineral interests noted in the table above includes the carrying value of our unproved oil & gas mineral interests totaling $340.5 million and $376.2 million, respectively. As discussed in Note 2 – Summary of Significant Accounting Policies, we generally do not record depletion expense for our unproved oil & gas mineral interests; however, we do review for impairment as needed throughout the year. During 2020 and 2019, we incurred $13.1 million and $13.2 million, respectively, in mine development costs, primarily related to the development of our Excel Mine No. 5 at our MC Mining complex. All past capitalized mine development costs are associated with other mines that shifted to the production phase in past years and we are amortizing these costs accordingly. We believe that the carrying value of the past development costs will be recovered. For information regarding long-lived asset impairments please see Note 4 – Long-Lived Asset Impairments. See Note 2 – Summary of Significant Accounting Policies for more information on our accounting policy for property, plant and equipment.

LONG-TERM DEBT

LONG-TERM DEBT	12 Months Ended
LONG-TERM DEBT	Dec. 31, 2020
LONG-TERM DEBT
LONG-TERM DEBT	8. LONG-TERM DEBT Long-term debt consists of the following: Unamortized Discount and Principal Debt Issuance Costs December 31, December 31, 2020 2019 2020 2019 (in thousands) Revolving credit facility $ 87,500 $ 255,000 $ (7,196) $ (3,050) Senior notes 400,000 400,000 (3,964) (4,879) Securitization facility 55,900 73,800 — — May 2019 equipment financing 4,956 8,199 — — November 2019 equipment financing 42,367 52,281 — — June 2020 equipment financing 13,057 — — — 603,780 789,280 (11,160) (7,929) Less current maturities (73,199) (13,157) — — Total long-term debt $ 530,581 $ 776,123 $ (11,160) $ (7,929) Credit Facility. The Credit Agreement is guaranteed by certain of our Intermediate Partnership's material direct and indirect subsidiaries (the "Restricted Subsidiaries") and is secured by substantially all of the assets of the Restricted Subsidiaries. The Credit Agreement is also guaranteed by Alliance Minerals but the oil and gas minerals assets of Alliance Minerals and its direct and indirect subsidiaries (collectively with Alliance Minerals, the "Unrestricted Subsidiaries") are not collateral under the Credit Agreement. Borrowings under the Revolving Credit Facility bear interest, at our option, at either (i) the Base Rate at the greater of three benchmarks or (ii) a Eurodollar Rate, plus margins for (i) or (ii), as applicable, that fluctuate depending upon the ratio of Consolidated Debt to Consolidated Cash Flow (each as defined in the Credit Agreement). The Eurodollar Rate, with applicable margin, under the Revolving Credit Facility was 3.01% as of December 31, 2020. At December 31, 2020, we had $21.8 million of letters of credit outstanding with $428.5 million available for borrowing under the Revolving Credit Facility. We incur an annual commitment fee of 0.35% on the undrawn portion of the Revolving Credit Facility. We utilize the Revolving Credit Facility, as appropriate, for working capital requirements, capital expenditures and investments, scheduled debt payments and distribution payments. The Credit Agreement contains various restrictions affecting the Intermediate Partnership and its Restricted Subsidiaries including, among other things, restrictions on incurrence of additional indebtedness and liens, sale of assets, investments, mergers and consolidations and transactions with affiliates, including transactions with Unrestricted Subsidiaries. In each case, these restrictions are subject to various exceptions. In addition, the payment of cash distributions is restricted if such payment would result in a fixed charge coverage ratio of less than 1.0 to 1.0 (as defined in the Credit Agreement) for the four most recently ended fiscal quarters. The Credit Agreement requires the Intermediate Partnership to maintain (a) a debt to cash flow ratio of not more than 2.5 to 1.0, (b) a cash flow to interest expense ratio of not less than 3.0 to 1.0 and (c) a first lien debt to cash flow ratio of not more than 1.5 to 1.0, in each case, during the four most recently ended fiscal quarters. The debt to cash flow ratio, cash flow to interest expense ratio and first lien debt to cash flow ratio were 1.53 to 1.0, 8.45 to 1.0 and 0.52 to 1.0, respectively, for the trailing twelve months ended December 31, 2020. We remained in compliance with the covenants of the Credit Agreement as of December 31, 2020 and anticipate remaining in compliance with the covenants. Net restricted assets, as defined by the Securities and Exchange Commission, refers to the amount of our consolidated subsidiaries’ net assets for which the ability to transfer funds to ARLP in the form of cash dividends, loans, advances, or transfers is restricted. As a result of the restrictions contained in the Credit Agreement and our current compliance ratios, the amount of our net restricted assets at December 31, 2020, was $240.8 million. Senior Notes. Accounts Receivable Securitization May 2019 Equipment Financing. November 2019 Equipment Financing. June 2020 Equipment Financing. Other. Aggregate maturities of long-term debt are payable as follows: Year Ended December 31, (in thousands) 2021 $ 73,199 2022 16,071 2023 24,970 2024 89,540 2025 400,000 $ 603,780

LEASES

LEASES	12 Months Ended
LEASES	Dec. 31, 2020
LEASES
LEASES	9. LEASES The components of lease expense were as follows: December 31, 2020 2019 (in thousands) Finance lease cost: $ Amortization of right-of-use assets $ 704 14,608 Interest on lease liabilities 377 2,085 Operating lease cost 3,873 9,169 Short-term lease cost 84 464 Variable lease cost 1,375 1,360 Total lease cost $ 6,413 $ 27,686 Rental expense was $5.2 million, $11.0 million and $14.9 million for the years ended December 31, 2020, 2019 and 2018, respectively. Supplemental cash flow information related to leases was as follows: December 31, 2020 2019 (in thousands) Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows for operating leases $ 3,870 9,124 Operating cash flows for finance leases $ 377 891 Financing cash flows for finance leases $ 8,368 46,725 Right-of-use assets obtained in exchange for lease obligations: Operating leases $ 278 25,593 Supplemental balance sheet information related to leases was as follows: December 31, 2020 2019 (in thousands) Finance leases: Property and equipment finance lease assets, gross $ 5,485 $ 30,610 Accumulated depreciation (3,867) (20,564) Property and equipment finance lease assets, net $ 1,618 $ 10,046 December 31, 2020 2019 Weighted average remaining lease term Operating leases 13.4 years 13.1 years Finance leases 3.9 years 1.6 years Weighted average discount rate Operating leases 6.0 % 6.0 % Finance leases 8.0 % 6.0 % Maturities of lease liabilities as of December 31, 2020 were as follows: Operating leases Finance leases (in thousands) 2021 $ 2,346 $ 912 2022 2,245 912 2023 2,061 139 2024 1,841 139 2025 1,527 139 Thereafter 11,838 280 Total lease payments 21,858 2,521 Less imputed interest (6,926) (297) Total $ 14,932 $ 2,224

FAIR VALUE MEASUREMENTS

FAIR VALUE MEASUREMENTS	12 Months Ended
FAIR VALUE MEASUREMENTS	Dec. 31, 2020
FAIR VALUE MEASUREMENTS
FAIR VALUE MEASUREMENTS	10. FAIR VALUE MEASUREMENTS The following table summarizes our fair value measurements within the hierarchy not included elsewhere in these notes: December 31, 2020 December 31, 2019 Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 (in thousands) Long-term debt $ — $ 518,317 $ — $ — $ 736,206 $ — Total $ — $ 518,317 $ — $ — $ 736,206 $ — See Note 2 – Summary of Significant Accounting Policies – Fair Value Measurements for more information regarding fair value hierarchy levels. The carrying amounts for cash equivalents, accounts receivable, accounts payable, accrued and other liabilities, due from affiliates and due to affiliates approximate fair value due to the short maturity of those instruments. The estimated fair value of our long-term debt, including current maturities, is based on interest rates that we believe are currently available to us in active markets for issuance of debt with similar terms and remaining maturities (See Note 8 – Long-Term Debt). The fair value of debt, which is based upon these interest rates, is classified as a Level 2 measurement under the fair value hierarchy.

PARTNERS' CAPITAL

PARTNERS' CAPITAL	12 Months Ended
PARTNERS' CAPITAL	Dec. 31, 2020
PARTNERS' CAPITAL
PARTNERS' CAPITAL	11. PARTNERS' CAPITAL Distributions Our available cash that is not used for unit repurchases may, at the discretion of our general partner, be distributed within 45 days after the end of each quarter to unitholders of record. Available cash is generally defined in the partnership agreement as all cash and cash equivalents on hand at the end of each quarter less reserves established by MGP in its reasonable discretion for future cash requirements. These reserves are retained to provide for the conduct of our business, the payment of debt principal and interest and to provide funds for future distributions. The following table summarizes the quarterly per unit distribution paid during each quarter of 2018 through 2020: Year Ended December 31, 2020 2019 2018 First Quarter $ 0.400 $ 0.530 $ 0.510 Second Quarter $ — $ 0.535 $ 0.515 Third Quarter $ — $ 0.540 $ 0.520 Fourth Quarter $ — $ 0.540 $ 0.525 In response to the disruptions to the economy and the uncertainty surrounding the COVID-19 pandemic, the Board of Directors of ARLP's general partner began suspending cash distributions to unitholders with the First Quarter and has continued through the quarter ended December 31, 2020. Simplification Transactions On May 31, 2018, as part of the Simplification Transactions discussed in Note 1 – Organization and Presentation, ARLP issued 1,322,388 ARLP common units to the Owners of SGP in exchange for causing SGP to contribute to ARLP all of SGP's limited partner interests in AHGP, which included AHGP's indirect ownership of a 1.0001% general partner interest in the Intermediate Partnership and a 0.001% managing member interest in Alliance Coal. The Simplification Transactions are accounted for prospectively as an exchange of equity interests between entities under common control. Since ARLP and AHGP were under common control both before and after the Simplification Transactions, no fair value adjustment was made to the assets or liabilities of AHGP and its subsidiaries and no gain or loss was recognized on our consolidated financial statements. Unit Repurchase Program In May 2018, the Board of Directors approved the establishment of a unit repurchase program authorizing us to repurchase and retire up to $100 million of ARLP common units. The program has no time limit and we may repurchase units from time to time in the open market or in other privately negotiated transactions. The unit repurchase program authorization does not obligate us to repurchase any dollar amount or number of units. No unit repurchases were made during the year ended December 31, 2020. Since inception of the unit repurchase program, we have repurchased and retired 5,460,639 units at an average unit price of $17.12 for an aggregate purchase price of $93.5 million. Affiliated Entity Contributions An affiliated entity controlled by Mr. Craft made a capital contribution of $2.1 million during the year ended December 31, 2018 for the purpose of funding certain general and administrative expenses. On June 29, 2018, the members of this affiliated entity contributed 467,018 ARLP common units for similar purposes. Other The noncontrolling interest in our consolidated balance sheets represents Bluegrass Minerals' ownership interest in Cavalier Minerals. Our accumulated other comprehensive loss consists of unrecognized actuarial gains and losses as well as unrecognized prior service costs related to our pension and pneumoconiosis benefits. See Note 12 – Variable Interest Entities, Note 16 –Employee Benefit Plans and Note 20 – Accrued Workers' Compensation and Pneumoconiosis Benefits for further information.

VARIABLE INTEREST ENTITIES

VARIABLE INTEREST ENTITIES	12 Months Ended
VARIABLE INTEREST ENTITIES	Dec. 31, 2020
VARIABLE INTEREST ENTITIES
VARIABLE INTEREST ENTITIES	12. VARIABLE INTEREST ENTITIES Cavalier Minerals On November 10, 2014, our subsidiary, Alliance Minerals, and Bluegrass Minerals entered into a limited liability company agreement (the "Cavalier Agreement") to create Cavalier Minerals, which was formed to indirectly acquire oil & gas mineral interests through its ownership in AllDale I & II. Alliance Minerals owns a 96% member interest in Cavalier Minerals, and Bluegrass Minerals owns a 4% member interest in Cavalier Minerals and a profits interest which entitles it to receive distributions equal to 25% of all distributions (including in liquidation) after all members have recovered their investment. Distributions with respect to Bluegrass Minerals' profits interest will be offset by all distributions received by Bluegrass Minerals from the former general partners of AllDale I & II. To date, there has been no profits interest distribution. Bluegrass Minerals was Cavalier Minerals' managing member prior to the AllDale Acquisition (see Note 3 – Acquisitions). In conjunction with the AllDale Acquisition, we became the managing member in Cavalier Minerals. Total contributions to and cumulative distributions from Cavalier Minerals are as follows: Alliance Bluegrass Minerals Minerals (in thousands) Contributions $ 143,112 $ 5,963 Distributions 89,380 3,723 We have concluded that Cavalier Minerals is a VIE which we consolidate as the primary beneficiary because we are the managing member and a substantial equity owner in Cavalier Minerals. Bluegrass Minerals' equity ownership of Cavalier Minerals is accounted for as noncontrolling ownership interest in our consolidated balance sheets. In addition, earnings attributable to Bluegrass Minerals are recognized as noncontrolling interest in our consolidated statements of income. AllDale III In February 2017, Alliance Minerals committed to directly invest $30.0 million in AllDale III which was created for similar investment purposes as AllDale I & II. Alliance Minerals completed funding of this commitment in 2018. Alliance Minerals' limited partner interest in AllDale III at December 31, 2020 was 13.9%. The AllDale III Partnership Agreement includes a 25% profits interest for the general partner, subject to a return hurdle equal to the greater of 125% of cumulative capital contributions and a 10% internal rate of return, and following an 80/20 "catch-up" provision for the general partner. Since AllDale III is structured as a limited partnership with the limited partners 1) not having the ability to remove the general partner and 2) not participating significantly in the operational decisions, we concluded that AllDale III is a VIE. We are not the primary beneficiary of AllDale III as we do not have the power to direct the activities that most significantly impact AllDale III's economic performance. We account for our ownership interest in the income or loss of AllDale III as an equity method investment. We record equity income or loss based on AllDale III's distribution structure. See Note 13 – Investments for more information. WKY CoalPlay On November 17, 2014, SGP Land, LLC ("SGP Land"), a wholly owned subsidiary of SGP, and two limited liability companies ("Craft Companies") owned by irrevocable trusts established by Mr. Craft and his children entered into a limited liability company agreement to form WKY CoalPlay, LLC ("WKY CoalPlay"). WKY CoalPlay was formed, in part, to purchase and lease coal reserves. WKY CoalPlay is managed by one of the Craft Companies. In December 2014 and February 2015, we entered into various coal reserve leases with WKY CoalPlay. See Note 21 – Related-Party Transactions for further information on our lease terms with WKY CoalPlay. We concluded that WKY CoalPlay was a VIE because of our ability to exercise options to acquire reserves under lease with WKY CoalPlay (Note 21 – Related-Party Transactions), which was not within the control of the equity holders and, if it had occurred, could potentially limit the expected residual return to the owners of WKY CoalPlay. We hold no economic or governance rights related to WKY CoalPlay and our options did not give us any rights to impact WKY CoalPlay's economic performance. We therefore concluded that we were not the primary beneficiary of WKY CoalPlay. These options expired in December 2020 and February 2021. Upon the expiration of these options, WKY CoalPlay ceased to be a VIE. See Note 2 – Summary of Significant Accounting Policies for more information on our accounting policy for variable interest entities.

INVESTMENTS

INVESTMENTS	12 Months Ended
INVESTMENTS	Dec. 31, 2020
INVESTMENTS
INVESTMENTS	13. INVESTMENTS AllDale III As discussed in Note 12 – Variable Interest Entities, we account for our ownership interest in the income or loss of AllDale III as an equity method investment. We record equity income or loss based on AllDale III's distribution structure. The changes in our equity method investment in AllDale III for each of the periods presented were as follows: Year Ended December 31, 2020 2019 2018 (in thousands) Beginning balance $ 28,529 $ 28,974 $ 14,182 Contributions — — 15,600 Equity method investment income 907 2,203 547 Distributions received (1,895) (2,648) (1,355) Other (273) — — Ending balance $ 27,268 $ 28,529 $ 28,974 As discussed in Note 4 – Long-Lived Asset Impairments, there was uncertainty related to energy demand in the First Quarter as a result of weak electricity demand and an oversupply and lack of storage for oil and natural gas, both due in part to the COVID-19 pandemic and other market and production factors, which could have impacted our investment in AllDale III. As a result, as part of our First Quarter impairment assessment, we compared the fair value of our investment to its carrying value and concluded that the fair value exceeded the carrying value and no impairment in our investment was necessary. In our subsequent impairment assessments, amid a modest recovery in commodity futures prices and increased clarity into production levels by operators during the year, we again compared the fair value of our investment to its carrying value and concluded no impairment was necessary. To calculate the fair value of the investment we used an income approach utilizing a discounted cash flow model based on our estimate of both production, prices and expenses from information available to us. Key assumptions used in our valuation are not observable in active markets; therefore, the fair value measurements represent Level 3 fair value measurements. The cash flow estimates used in our assessments, by their very nature, are dependent on conditions that could materially change in future periods based on new information. If in future periods changes to these estimates were to materially reduce our expected cash flows, an impairment of our investment could be necessary. Kodiak On July 19, 2017, Alliance Minerals purchased $100 million of Series A-1 Preferred Interests from Kodiak, a privately-held company providing large-scale, high-utilization gas compression assets to customers operating primarily in the Permian Basin. This structured investment provided us with a quarterly cash or payment-in-kind return. On February 8, 2019, Kodiak redeemed our preferred interest for $135.0 million in cash resulting in an $11.5 million gain due to an early redemption premium. The gain is included in the Equity securities income See Note 2 – Summary of Significant Accounting Policies for more information on our accounting policy for investments.

REVENUE FROM CONTRACTS WITH CUS

REVENUE FROM CONTRACTS WITH CUSTOMERS	12 Months Ended
REVENUE FROM CONTRACTS WITH CUSTOMERS	Dec. 31, 2020
REVENUE FROM CONTRACTS WITH CUSTOMERS
REVENUE FROM CONTRACTS WITH CUSTOMERS	14. REVENUE FROM CONTRACTS WITH CUSTOMERS The following table illustrates the disaggregation of our revenues by type, including a reconciliation to our segment presentation as presented in Note 24 – Segment Information. Illinois Other and Basin Appalachia Minerals Corporate Elimination Consolidated (in thousands) Year Ended December 31, 2020 Coal sales $ 755,208 $ 477,064 $ — $ — $ — $ 1,232,272 Oil & gas royalties — — 42,912 — — 42,912 Transportation revenues 12,817 8,312 — — — 21,129 Other revenues 2,026 14,954 229 25,124 (10,517) 31,816 Total revenues $ 770,051 $ 500,330 $ 43,141 $ 25,124 $ (10,517) $ 1,328,129 Year Ended December 31, 2019 Coal sales $ 1,128,588 $ 628,406 $ — $ 22,138 $ (16,690) $ 1,762,442 Oil & gas royalties — — 51,735 — — 51,735 Transportation revenues 94,686 4,817 — — — 99,503 Other revenues 13,034 11,166 1,301 34,712 (12,173) 48,040 Total revenues $ 1,236,308 $ 644,389 $ 53,036 $ 56,850 $ (28,863) $ 1,961,720 Year Ended December 31, 2018 Coal sales $ 1,197,143 $ 635,530 $ — $ 43,393 $ (31,258) $ 1,844,808 Transportation revenues 106,947 5,435 — 3 — 112,385 Other revenues 16,999 3,000 — 38,096 (12,431) 45,664 Total revenues $ 1,321,089 $ 643,965 $ — $ 81,492 $ (43,689) $ 2,002,857 The following table illustrates the amount of our transaction price for all current coal supply contracts allocated to performance obligations that are unsatisfied or partially unsatisfied as of December 31, 2020 and disaggregated by segment and contract duration. 2024 and 2021 2022 2023 Thereafter Total (in thousands) Illinois Basin coal revenues $ 653,208 $ 253,654 $ 187,570 $ 140,750 $ 1,235,182 Appalachia coal revenues 318,984 95,471 — — 414,455 Total coal revenues (1) $ 972,192 $ 349,125 $ 187,570 $ 140,750 $ 1,649,637 (1) Coal revenues generally consists of consolidated revenues excluding our Minerals segment.

EARNINGS PER LIMITED PARTNER UN

EARNINGS PER LIMITED PARTNER UNIT	12 Months Ended
EARNINGS PER LIMITED PARTNER UNIT	Dec. 31, 2020
EARNINGS PER LIMITED PARTNER UNIT
EARNINGS PER LIMITED PARTNER UNIT	15. EARNINGS PER LIMITED PARTNER UNIT We utilize the two-class method in calculating basic and diluted earnings per limited partner unit ("EPU"). Subsequent to the Simplification Transactions, net income attributable to ARLP is only allocated to limited partners and participating securities under deferred compensation plans. Net losses attributable to ARLP are allocated to limited partners but not to participating securities. Prior to the Simplification Transactions, net income attributable to ARLP was allocated to our general partner, limited partners and participating securities under deferred compensation plans in accordance with their respective partnership ownership percentages. As a result of the Simplification Transactions, MGP no longer holds economic interests in the Intermediate Partnership or Alliance Coal. We currently do not make distributions or allocate income and losses to MGP in our calculation of EPU. Please see Note 1 – Organization and Presentation for more information on the Simplification Transactions. Our participating securities under deferred compensation plans include rights to nonforfeitable distributions or distribution equivalents. Our participating securities are outstanding awards under our LTIP and phantom units in notional accounts under our SERP and the Directors' Deferred Compensation Plan. The following is a reconciliation of net income (loss) attributable to ARLP used for calculating basic and diluted earnings per unit and the weighted-average units used in computing EPU. Year Ended December 31, 2020 2019 2018 (in thousands, except per unit data) Net income (loss) attributable to ARLP $ (129,220) $ 399,414 $ 366,604 Adjustment: General partner's equity ownership (1) — — (1,560) Limited partners' interest in net income (loss) attributable to ARLP (129,220) 399,414 365,044 Less: Distributions to participating securities — (4,254) (5,114) Undistributed earnings attributable to participating securities — (2,237) (1,641) Net income (loss) attributable to ARLP available to limited partners $ (129,220) $ 392,923 $ 358,289 Weighted-average limited partner units outstanding – basic and diluted 127,165 128,117 130,758 Earnings per limited partner unit - basic and diluted (2) $ (1.02) $ 3.07 $ 2.74 (1) Amounts presented for periods subsequent to the first quarter of 2018 reflect the impact of the Simplification Transactions, which ended net income allocations and quarterly cash distributions to MGP after May 31, 2018. Prior to the Simplification Transactions, MGP maintained a 1.0001% general partner interest in the Intermediate Partnership and a 0.001% managing member interest in Alliance Coal and thus received quarterly distributions and income and loss allocations during this time period. (2) Diluted EPU gives effect to all potentially dilutive common units outstanding during the period using the treasury stock method. Diluted EPU excludes all potentially dilutive units calculated under the treasury stock method if their effect is anti-dilutive. For the years ended December 31, 2020, 2019 and 2018, the combined total of LTIP, SERP and Directors' Deferred Compensation Plan units of 773,664 , 1,284,013 and 1,658,908 , respectively, were considered anti-dilutive under the treasury stock method.

EMPLOYEE BENEFIT PLANS

EMPLOYEE BENEFIT PLANS	12 Months Ended
EMPLOYEE BENEFIT PLANS	Dec. 31, 2020
EMPLOYEE BENEFIT PLANS
EMPLOYEE BENEFIT PLANS	16. EMPLOYEE BENEFIT PLANS Defined Contribution Plans Defined Benefit Plan The following sets forth changes in benefit obligations and plan assets for the years ended December 31, 2020 and 2019 and the funded status of the Pension Plan reconciled with the amounts reported in our consolidated financial statements: December 31, 2020 2019 (dollars in thousands) Change in benefit obligations: Benefit obligations at beginning of year $ 136,425 $ 118,958 Interest cost 4,185 4,864 Actuarial loss 12,396 17,084 Benefits paid (5,072) (4,481) Benefit obligations at end of year 147,934 136,425 Change in plan assets: Fair value of plan assets at beginning of year 91,567 75,823 Employer contribution 1,739 5,559 Actual return on plan assets 12,735 14,666 Benefits paid (5,072) (4,481) Fair value of plan assets at end of year 100,969 91,567 Funded status at the end of year $ (46,965) $ (44,858) Amounts recognized in balance sheet: Non-current liability $ (46,965) $ (44,858) Amounts recognized in accumulated other comprehensive income consists of: Prior service cost $ (754) $ (940) Net actuarial loss (46,519) (45,125) $ (47,273) $ (46,065) Weighted-average assumption to determine benefit obligations as of December 31, Discount rate 2.37% 3.15% Weighted-average assumptions used to determine net periodic benefit cost for the year ended December 31, Discount rate 3.15% 4.17% Expected return on plan assets 6.50% 6.50% The actuarial loss components of the change in benefit obligations in 2020 and 2019 were primarily attributable to decreases in the discount rate compared to the prior year-end, offset in part by updated mortality tables. The expected long-term rate of return used to determine our pension liability is based on a 1.5% active management premium in addition to an asset allocation assumption of: Asset allocation As of December 31, 2020 assumption Equity securities 62% Fixed income securities 33% Real estate 5% 100% The actual return on plan assets was 14.2% and 19.2% for the years ended December 31, 2020 and 2019, respectively. Year Ended December 31, 2020 2019 2018 (in thousands) Components of net periodic benefit cost: Interest cost $ 4,185 $ 4,864 $ 4,462 Expected return on plan assets (5,861) (4,932) (5,784) Amortization of prior service cost 186 186 186 Amortization of net loss 4,128 3,922 3,608 Net periodic benefit cost (1) $ 2,638 $ 4,040 $ 2,472 (1) Nonservice components of net periodic benefit cost are included in the Other income (expense) line item within our consolidated statements of income. Year Ended December 31, 2020 2019 (in thousands) Other changes in plan assets and benefit obligation recognized in accumulated other comprehensive loss: Net actuarial loss $ (5,522) $ (7,350) Reversal of amortization item: Prior service cost 186 186 Net actuarial loss 4,128 3,922 Total recognized in accumulated other comprehensive loss (1,208) (3,242) Net periodic benefit cost (2,638) (4,040) Total recognized in net periodic benefit cost and accumulated other comprehensive loss $ (3,846) $ (7,282) Estimated future benefit payments as of December 31, 2020 are as follows: Year Ended December 31, (in thousands) 2021 $ 5,629 2022 5,954 2023 6,269 2024 6,488 2025 6,620 2026-2030 34,674 $ 65,634 We expect to contribute $6.5 million to the Pension Plan in 2021. The Compensation Committee has appointed an investment manager with full investment authority with respect to Pension Plan investments subject to investment guidelines and compliance with ERISA or other applicable laws. The investment manager employs a series of asset allocation strategy phases to glide the portfolio risk commensurate with both plan characteristics and market conditions. The objective of the allocation policy is to reach and maintain fully funded status. The total portfolio allocation will be adjusted as the funded ratio of the Pension Plan changes and market conditions warrant. The target allocation includes investments in equity and fixed income commingled investment funds. Total account performance is reviewed at least annually, using a dynamic benchmark approach to track investment performance. General asset allocation guidelines at December 31, 2020 are as follows: Percentage of Total Portfolio Minimum Target Maximum Equity securities 45% 62% 80% Fixed income securities 10% 33% 55% Real estate 0% 5% 10% Equity securities include domestic equity securities, developed international securities, emerging markets equity securities and real estate investment trust. Fixed income securities include domestic and international investment grade fixed income securities, high yield securities and emerging markets fixed income securities. Fixed income futures may also be utilized within the fixed income securities asset allocation. The following information discloses the fair values of our Pension Plan assets by asset category: December 31, 2020 2019 (in thousands) Cash and cash equivalents (a) $ 3,888 $ 2,958 Commingled investment funds measured at net asset value (b): Equities - Global 17,549 10,028 Equities - United States 31,835 26,812 Equities - United States futures (2,616) — Equities - International developed markets 8,920 10,528 Equities - International developed markets futures (4,921) — Equities - International emerging markets 6,600 8,410 Equities - International emerging markets futures (975) — Fixed income - Investment grade 25,703 26,186 Fixed income - High yield 10,056 — Fixed income - Emerging markets 2,664 — Fixed income - Futures (1,265) — Real estate 3,531 4,355 Other — 2,290 Total $ 100,969 $ 91,567 (a) Cash and cash equivalents represents a Level 1 fair value measurement. See Note 2 – Summary of Significant Accounting Policies – Fair Value Measurements for more information regarding the definitions of fair value hierarchy levels. (b) Investments measured at fair value using the net asset value per share (or its equivalent) have not been classified within the fair value hierarchy. The fair values of all commingled investment funds are determined based on the net asset values per unit of each of the funds. The net asset values per unit represent the aggregate value of the fund's assets at fair value less liabilities, divided by the number of units outstanding. See Note 2 – Summary of Significant Accounting Policies for more information on our accounting policy for pension benefits.

COMMON UNIT-BASED COMPENSATION

COMMON UNIT-BASED COMPENSATION PLANS	12 Months Ended
COMMON UNIT-BASED COMPENSATION PLANS	Dec. 31, 2020
COMMON UNIT-BASED COMPENSATION PLANS
COMMON UNIT-BASED COMPENSATION PLANS	17. COMMON UNIT-BASED COMPENSATION PLANS Long-Term Incentive Plan We maintain the LTIP for certain employees and officers of MGP and its affiliates who perform services for us. As part of our LTIP, unit awards of non-vested "phantom" or notional units, also referred to as "restricted units", may be granted which upon satisfaction of time and performance-based vesting requirements, entitle the LTIP participant to receive ARLP common units. Annual grant levels and vesting provisions of restricted units for designated participants are recommended by Mr. Craft, subject to review and approval of the Compensation Committee. Vesting of all restricted units outstanding is subject to the satisfaction of certain financial tests. If it is not probable the financial tests for a particular grant of restricted units will be met, any previously expensed amounts for that grant are reversed and no future expense will be recognized for that grant. Assuming the financial tests are met, grants of restricted units issued to LTIP participants are generally expected to cliff vest on January 1st of the third year following issuance of the grants. We expect to settle restricted unit grants by delivery of ARLP common units, except for the portion of the grants that will satisfy employee tax withholding obligations of LTIP participants. We account for forfeitures of non-vested LTIP restricted unit grants as they occur. As provided under the DERs provisions of the LTIP and the terms of the LTIP restricted unit awards, all non-vested restricted units include contingent rights to receive quarterly distributions in cash or, at the discretion of the Compensation Committee, phantom units in lieu of cash credited to a bookkeeping account with value equal to the cash distributions we make to unitholders during the vesting period. If it is not probable the financial tests for a particular grant of restricted units will be met, any previously paid DER amounts for that grant are reversed from Partners’ Capital and recorded as compensation expense and any future DERs, for that grant, if any, will be recognized as compensation expense when paid. A summary of non-vested LTIP grants of restricted units is as follows: Number of units Weighted average grant date fair value per unit Intrinsic value (in thousands) Non-vested grants at January 1, 2018 1,694,026 $ 19.62 $ 33,372 Granted 511,305 20.40 Vested (1) (331,502) 34.61 Forfeited (45,749) 17.40 Non-vested grants at December 31, 2018 1,828,080 17.18 31,699 Granted 682,155 18.63 Vested (1) (885,381) 12.38 Forfeited (21,476) 20.84 Non-vested grants at December 31, 2019 1,603,378 20.39 17,349 Granted (2) 1,430,489 5.02 Vested (3) (919,524) 21.70 Grants canceled (4) (675,302) 18.62 Forfeited (8,552) 20.16 Non-vested grants at December 31, 2020 1,430,489 5.02 6,409 (1) During the years ended December 31, 2019 and 2018, we issued 596,650 and 191,858 , respectively, unrestricted common units to LTIP participants. The remaining vested units were settled in cash to satisfy tax withholding obligations of the LTIP participants. (2) In December 2020, we modified the vesting requirements for certain restricted units that we granted in February 2020 which were determined to be improbable of vesting under the original vesting requirements (the "2020 Grants"). The new vesting requirements make it probable the modified restricted units will vest. Also in December 2020, an additional 578,114 restricted units under these modified vesting requirements were granted. The grant date fair value reflects the modification date fair value for those awards that were modified. (3) In February 2020, we issued 279,622 unrestricted common units to LTIP participants as a result of satisfying the vesting requirements for 424,486 restricted units that were granted in 2017. The remaining vested units were settled in cash to satisfy tax withholding obligations of the LTIP participants. In December 2020, we accelerated the vesting requirements for 495,038 restricted units that were granted in 2018 (the "2018 Grants") and settled these restricted units in cash. (4) In December 2020, 675,302 restricted units that were granted in 2019 (the "2019 Grants") were canceled since it was determined that the vesting requirements for these restricted units were not probable of being satisfied. For the years ended December 31, 2020, 2019 and 2018, our LTIP expense for grants of restricted units was $8.1 million, $10.4 million and $10.8 million, respectively. LTIP expense for grants of restricted units for the year ended December 31, 2020 includes the impact of the reversal of the 2019 Grants, the modification of the 2020 Grants and incremental compensation cost associated with the cash settlement of the 2018 Grants. The cash settlement of the 2018 Grants was the first time we have settled restricted units in cash and we currently do not expect to do so again in the future. The cash settlement of the 2018 Grants resulted in $5.4 million in incremental compensation cost. The 2019 Grants were determined to be not probable of vesting therefore $4.8 million of cumulative previously recognized expense was reversed in 2020, offset in part by related DERs for the 2019 Grants previously recorded to equity and then expensed in 2020. The 2020 Grants were determined to be improbable of vesting therefore the Compensation Committee modified the awards to change the vesting requirement, which made the grants probable of vesting, and granted additional restricted units under these modified vesting requirements as previously discussed. As a result, the grant date fair value of the modified awards was changed to reflect the modification date fair value of the awards resulting in a net reduction in LTIP expense of $1.0 million for the year ended, December 31, 2020. The total obligation associated with LTIP grants of restricted units as of December 31, 2020 and 2019 was $1.3 million and $20.2 million, respectively, and is included in the partners' capital Limited partners-common unitholders Approximately 1.7 million units remain available under the LTIP for issuance in the future, assuming all grants currently issued and outstanding are settled with common units, without reduction for tax withholding, no future forfeitures occur and DERs are paid in cash versus additional phantom units. Supplemental Executive Retirement Plan and Directors' Deferred Compensation Plan We utilize the SERP to provide deferred compensation benefits for certain officers and key employees. All allocations made to participants under the SERP are made in the form of "phantom" ARLP units and SERP distributions will be settled in the form of ARLP common units. The SERP is administered by the Compensation Committee. Our directors participate in the Directors' Deferred Compensation Plan. Pursuant to the Directors' Deferred Compensation Plan, for amounts deferred either automatically or at the election of the director, a notional account is established and credited with notional common units of ARLP, described in the Directors' Deferred Compensation Plan as "phantom" units. Distributions from the Directors' Deferred Compensation Plan will be settled in the form of ARLP common units. For both the SERP and Directors' Deferred Compensation Plan, when quarterly cash distributions are made with respect to ARLP common units, an amount equal to such quarterly distribution is credited to each participant's notional account as additional phantom units. All grants of phantom units under the SERP and Directors' Deferred Compensation Plan vest immediately. A summary of SERP and Directors' Deferred Compensation Plan activity is as follows: Number of units Weighted average grant date fair value per unit Intrinsic value (in thousands) Phantom units outstanding as of January 1, 2018 561,784 $ 28.64 $ 11,067 Granted 84,417 18.78 Issued (1) (10,364) 27.92 Phantom units outstanding as of December 31, 2018 635,837 27.34 11,025 Granted 111,012 14.50 Issued (1) (115,484) 25.20 Phantom units outstanding as of December 31, 2019 631,365 25.48 6,831 Granted 129,265 5.25 Phantom units outstanding as of December 31, 2020 760,630 22.04 3,408 (1) During the years ended December 31, 2019 and 2018, we issued ARLP common units of 115,484 and 7,181 , respectively, to participants under the SERP and Directors' Deferred Compensation Plan . Units issued in 2018 were net of units settled in cash to satisfy tax withholding obligations. Total SERP and Directors' Deferred Compensation Plan expense was $0.7 million, $1.6 million and $1.6 million for the years ended December 31, 2020, 2019 and 2018, respectively. As of December 31, 2020 and 2019, the total obligation associated with the SERP and Directors' Deferred Compensation Plan was $16.8 million and $16.1 million, respectively, and is included in the partners' capital Limited partners-common unitholders See Note 2 – Summary of Significant Accounting Policies for more information on our accounting policy for unit-based compensation.

SUPPLEMENTAL CASH FLOW INFORMAT

SUPPLEMENTAL CASH FLOW INFORMATION	12 Months Ended
SUPPLEMENTAL CASH FLOW INFORMATION	Dec. 31, 2020
SUPPLEMENTAL CASH FLOW INFORMATION
SUPPLEMENTAL CASH FLOW INFORMATION	18. SUPPLEMENTAL CASH FLOW INFORMATION Year Ended December 31, 2020 2019 2018 (in thousands) Cash Paid For: Interest $ 44,226 $ 43,093 $ 38,450 Income taxes $ 12 $ — $ 34 Non-Cash Activity: Accounts payable for purchase of property, plant and equipment $ 5,731 $ 14,504 $ 14,585 Right-of-use assets acquired by operating lease $ 278 25,593 — Market value of common units issued under deferred compensation plans before tax withholding requirements $ 3,837 $ 17,415 $ 6,142

ASSET RETIREMENT OBLIGATIONS

ASSET RETIREMENT OBLIGATIONS	12 Months Ended
ASSET RETIREMENT OBLIGATIONS	Dec. 31, 2020
ASSET RETIREMENT OBLIGATIONS
ASSET RETIREMENT OBLIGATIONS	19. ASSET RETIREMENT OBLIGATIONS The majority of our operations are governed by various state statutes and the Federal Surface Mining Control and Reclamation Act of 1977, which establish reclamation and mine closing standards. These regulations require, among other things, restoration of property in accordance with specified standards and an approved reclamation plan. The following table presents the activity affecting the asset retirement and mine closing liability: Year Ended December 31, 2020 2019 (in thousands) Beginning balance $ 137,514 $ 137,114 Accretion expense 4,033 4,087 Payments (1,769) (2,948) Allocation of liability associated with acquisitions, mine development and change in assumptions (11,880) (739) Ending balance $ 127,898 $ 137,514 For the year ended December 31, 2020, the allocation of liability associated with acquisition, mine development and change in assumptions was a net decrease of $11.9 million. This net decrease was attributable to lower cost assumptions and completion of certain reclamation obligations across all operations, permit modifications and extension of projected mine life estimates at certain mines, partially offset by acquisition of property with existing reclamation liabilities. For the year ended December 31, 2019, the allocation of liability associated with acquisition, mine development and change in assumptions was immaterial. The impact of discounting our estimated cash flows resulted in reducing the accrual for asset retirement obligations by $102.1 million and $102.9 million at December 31, 2020 and 2019, respectively. Estimated payments of asset retirement obligations as of December 31, 2020 are as follows: Year Ended December 31, (in thousands) 2021 $ 6,411 2022 2,723 2023 2,570 2024 3,317 2025 4,601 Thereafter 210,330 Aggregate undiscounted asset retirement obligations 229,952 Effect of discounting (102,054) Total asset retirement obligations 127,898 Less: current portion (6,411) Non-current asset retirement obligations $ 121,487 Federal and state laws require bonds to secure our obligations to reclaim lands used for mining and are typically renewable on a yearly basis. As of December 31, 2020 and 2019, we had approximately $171.1 million and $181.6 million, respectively, in surety bonds outstanding to secure the performance of our reclamation obligations. See Note 2 – Summary of Significant Accounting Policies for more information on our accounting policy for asset retirement obligations.

ACCRUED WORKERS' COMPENSATION A

ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS	12 Months Ended
ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS	Dec. 31, 2020
ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS
ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS	20. ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS We provide income replacement and medical treatment for work-related traumatic injury claims as required by applicable state laws. Workers' compensation laws also compensate survivors of workers who suffer employment related deaths. Certain of our mine operating entities are liable under state statutes and the Federal Coal Mine Health and Safety Act of 1969, as amended, to pay benefits for black lung disease (or pneumoconiosis) to eligible employees and former employees and their dependents. Both pneumoconiosis and traumatic claims are covered through our self-insured programs. The following is a reconciliation of the changes in workers' compensation liability (including current and long-term liability balances): December 31, 2020 2019 (in thousands) Beginning balance $ 53,384 $ 49,539 Accruals increase 5,146 7,162 Payments (8,482) (11,320) Interest accretion 1,278 1,606 Valuation loss 3,413 6,397 Ending balance $ 54,739 $ 53,384 The discount rate used to calculate the estimated present value of future obligations for workers' compensation was 1.95% and 2.81% at December 31, 2020 and 2019, respectively. The valuation losses in both 2020 and 2019 were primarily attributable to a decrease in the discount rate used to calculate the estimated present value of future obligations as well as unfavorable changes in claims development in their respective years. As of December 31, 2020 and 2019, we had $95.2 million and $90.2 million, respectively, in surety bonds and letters of credit outstanding to secure workers' compensation obligations. We limit our exposure to traumatic injury claims by purchasing a high deductible insurance policy that starts paying benefits after deductibles for the particular claim year have been met. Our workers' compensation liability above is presented on a gross basis and does not include our expected receivables on our insurance policy. Our receivables for traumatic injury claims under this policy as of December 31, 2020 and 2019 are $7.1 million and $7.7 million, respectively. Our receivables are included in Other long-term assets The following is a reconciliation of the changes in pneumoconiosis benefit obligations: December 31, 2020 2019 (in thousands) Benefit obligations at beginning of year $ 97,683 $ 72,095 Service cost 3,526 2,593 Interest cost 2,998 3,044 Actuarial (gain) loss 7,787 23,298 Benefits and expenses paid (3,498) (3,347) Benefit obligations at end of year $ 108,496 $ 97,683 The following is a reconciliation of the changes in the pneumoconiosis benefit obligation recognized in accumulated other comprehensive loss: Year Ended December 31, 2020 2019 2018 (in thousands) Net actuarial gain (loss) $ (7,787) $ (23,298) $ 4,599 Reversal of amortization item: Net actuarial (gain) loss (686) (4,582) 2 Total recognized in accumulated other comprehensive loss $ (8,473) $ (27,880) $ 4,601 The discount rate used to calculate the estimated present value of future obligations for pneumoconiosis benefits was 2.38%, 3.12% and 4.13% at December 31, 2020, 2019 and 2018, respectively. Year Ended December 31, 2020 2019 2018 (in thousands) Amount recognized in accumulated other comprehensive loss consists of: Net actuarial loss $ 40,399 $ 31,927 $ 4,047 The actuarial loss component of the change in benefit obligations in 2020 was primarily attributable to a) a decrease in the discount rate used to calculate the estimated present value of the future obligations and b) an increase in the assumptions regarding future medical benefits and legal expenses. These components were partially offset in part by favorable demographic changes in the at-risk population. The actuarial loss component of the change in benefit obligations in 2019 was primarily attributable to a) a decrease in the discount rate used to calculate the estimated present value of the future obligations and b) an increase in Federal and State benefit levels. These components were offset in part by favorable demographic changes in the at-risk population. Summarized below is information about the amounts recognized in the accompanying consolidated balance sheets for pneumoconiosis and workers' compensation benefits: December 31, 2020 2019 (in thousands) Workers’ compensation claims $ 54,739 $ 53,384 Pneumoconiosis benefit claims 108,496 97,683 Total obligations 163,235 151,067 Less current portion (10,646) (11,175) Non-current obligations $ 152,589 $ 139,892 Both the pneumoconiosis benefit and workers' compensation obligations were unfunded at December 31, 2020 and 2019. The pneumoconiosis benefit and workers' compensation expense consists of the following components: Year Ended December 31, 2020 2019 2018 (in thousands) Black lung benefits: Service cost $ 3,526 $ 2,593 $ 2,525 Interest cost (1) 2,998 3,044 2,542 Net amortization (1) (686) (4,582) 2 Total pneumoconiosis expense 5,838 1,055 5,069 Workers' compensation expense 12,305 17,541 11,270 Net periodic benefit cost $ 18,143 $ 18,596 $ 16,339 ________________________________________ (1) Interest cost and net amortization is included in the Other income (expense) line item within our consolidated statements of income ( see Note 2 – Summary of Significant Accounting Policies). See Note 2 – Summary of Significant Accounting Policies for more information on our accounting policy for workers' compensation and pneumoconiosis benefits.

RELATED-PARTY TRANSACTIONS

RELATED-PARTY TRANSACTIONS	12 Months Ended
RELATED-PARTY TRANSACTIONS	Dec. 31, 2020
RELATED-PARTY TRANSACTIONS
RELATED-PARTY TRANSACTIONS	21. RELATED-PARTY TRANSACTIONS We have continuing related-party transactions with MGP and its affiliates. The Board of Directors and its conflicts committee ("Conflicts Committee") review our related-party transactions that involve a potential conflict of interest between our general partner or its affiliates and ARLP or its subsidiaries or any other partner of ARLP to determine that such transactions are fair and reasonable to ARLP. As a result of these reviews, the Board of Directors and the Conflicts Committee approved each of the transactions described below that had such potential conflict of interest as fair and reasonable to ARLP. Affiliate Coal Lease Agreements The following table summarizes advanced royalties outstanding and related payments and recoupments under our affiliate coal lease agreements: WKY CoalPlay Towhead Webster Henderson WKY SGP/Craft Foundations Coal Coal Coal CoalPlay Henderson Henderson Tunnel & Union Webster Henderson & Union Ridge Counties, KY County, KY County, KY Counties, KY Total Acquired Acquired Acquired Acquired Acquired 2005 December 2014 December 2014 December 2014 February 2015 (in thousands) As of January 1, 2018 $ 3,000 $ 10,684 $ 5,356 $ 7,566 $ 6,387 $ 32,993 Payments — 3,597 2,570 2,520 2,131 10,818 Recoupment (3,000) (204) (31) — (36) (3,271) Unrecoupable — — (7,895) — — (7,895) As of December 31, 2018 — 14,077 — 10,086 8,482 32,645 Payments 4,500 3,597 2,568 2,521 2,131 15,317 Recoupment (3,000) (1,071) — — (107) (4,178) Unrecoupable — — (2,568) — — (2,568) As of December 31, 2019 1,500 16,603 — 12,607 10,506 41,216 Payments 3,000 3,597 2,568 2,522 2,132 13,819 Recoupment (3,000) (1,022) — — (56) (4,078) Unrecoupable — — (2,568) — — (2,568) As of December 31, 2020 $ 1,500 $ 19,178 $ — $ 15,129 $ 12,582 $ 48,389 SGP/Craft Foundations — one WKY CoalPlay — In December 2014, WKY CoalPlay's subsidiaries, Towhead Coal Reserves, LLC and Henderson Coal Reserves, LLC entered into coal lease agreements with Alliance Resource Properties. The leases have initial terms of 20 years and provide for earned royalty payments of 4.0% of the coal sales price to both and annual minimum royalty payments of $3.6 million and $2.5 million, respectively. All annual minimum royalty payments for each agreement are recoupable from future earned royalties related to their respective agreements. Each agreement granted Alliance Resource Properties an option to acquire the leased reserves at any time during a three-year period beginning in December 2017 for a purchase price that would provide WKY CoalPlay a 7.0% internal rate of return on its investment in the reserves taking into account payments previously made under the leases. These options expired in December 2020. (See Note 12 – Variable Interest Entities). In December 2014, WKY CoalPlay's subsidiary, Webster Coal Reserves, LLC entered into a coal lease agreement with Alliance Resource Properties. The lease has an initial term of 7 years and provides for earned royalty payments of 4.0% of the coal sales price and annual minimum payments of $2.6 million. The agreement grants Alliance Resource Properties an option to acquire the leased reserves at any time during a three year period beginning in December 2017 for a purchase price that would provide WKY CoalPlay a 7.0% internal rate of return on its investment in the reserves taking into account payments previously made under the lease (See Note 12 – Variable Interest Entities). In the third quarter of 2019 it was determined that the balance of advanced royalties, the advance royalty payment in 2020 and the remaining advanced royalty payment expected in 2021 totaling $2.6 million, may not be recouped as a result of the reduction of the Dotiki’s economic mine life determined in 2018 and the subsequent ceasing of production in the third quarter of 2019. We accrued the expected future advance payments and recognized the charge in Asset Impairment expense in the third quarter of 2019. See Note 4 – Long-Lived Asset Impairments for more information. Cavalier Minerals

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
COMMITMENTS AND CONTINGENCIES	Dec. 31, 2020
COMMITMENTS AND CONTINGENCIES
COMMITMENTS AND CONTINGENCIES	22. COMMITMENTS AND CONTINGENCIES Commitments — Contractual Commitments — General Litigation — Various lawsuits, claims and regulatory proceedings incidental to our business are pending against the ARLP Partnership. We record an accrual for a potential loss related to these matters when, in management's opinion, such loss is probable and reasonably estimable. Based on known facts and circumstances, we believe the ultimate outcome of these outstanding lawsuits, claims and regulatory proceedings will not have a material adverse effect on our financial condition, results of operations or liquidity. However, if the results of these matters were different from management's current opinion and in amounts greater than our accruals, then they could have a material adverse effect. Other

CONCENTRATION OF CREDIT RISK AN

CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS	12 Months Ended
CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS	Dec. 31, 2020
CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS
CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS	23. CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS The international coal market has been a substantial part of our business with indirect sales to end-users in Europe, Africa, Asia, North America and South America. Our sales into the international coal market are considered exports and are made through brokered transactions. During the years ended December 31, 2020, 2019 and 2018, export tons represented approximately 3.3%, 17.9% and 27.8% of tons sold, respectively. We use the end-usage point as the basis for attributing tons to individual countries. Because title to our export shipments typically transfers to our brokerage customers at a point that does not necessarily reflect the end-usage point, we attribute export tons to the country with the end-usage point, if known. No individual country was attributed greater than 10% of total domestic and export tons sold during the years ended December 31, 2020, 2019 and 2018. We have significant long-term coal supply agreements, some of which contain prospective price adjustment provisions designed to reflect changes in market conditions, labor and other production costs and, in the infrequent circumstance when the coal is sold other than free on board the mine, changes in transportation rates. Our major customers are defined as those customers from which we derive at least ten percent of our total revenues, including transportation revenues. Total revenues from major customers are as follows: Year Ended December 31, Segment 2020 2019 2018 (in thousands) Customer A Illinois Basin $ 197,379 $ 228,500 $ 219,115 Customer B Appalachia — 213,319 — Customer C Illinois Basin 157,271 — — Customer D Illinois Basin/Appalachia 137,785 — — Trade accounts receivable from major customers totaled approximately $32.0 million and $26.3 million at December 31, 2020 and 2019, respectively. Our bad debt experience has historically been insignificant. Financial conditions of our customers could result in a material change to our bad debt expense in future periods. The coal supply agreements with these customers expire in 2022 for Customer C and Customer D and 2020 for Customer A.

SEGMENT INFORMATION

SEGMENT INFORMATION	12 Months Ended
SEGMENT INFORMATION	Dec. 31, 2020
SEGMENT INFORMATION
SEGMENT INFORMATION	24. SEGMENT INFORMATION We operate in the United States as a diversified natural resource company that generates income from the production and marketing of coal to major domestic and international utilities and industrial users as well as income from oil & gas mineral interests. We aggregate multiple operating segments into three reportable segments, Illinois Basin, Appalachia, and Minerals. We also have an "all other" category referred to as Other and Corporate. Our two coal reportable segments correspond to major coal producing regions in the eastern United States with similar economic characteristics including coal quality, geology, coal marketing opportunities, mining and transportation methods and regulatory issues. The two coal segments include seven mining complexes operating in Illinois, Indiana, Kentucky, Maryland, Pennsylvania and West Virginia and a coal loading terminal in Indiana on the Ohio River. The Minerals reportable segment aggregates our oil & gas mineral interests which are located primarily in the Permian (Delaware and Midland), Anadarko (SCOOP/STACK) and Williston (Bakken) basins. The operations within our Minerals reportable segment primarily include receiving royalties and lease bonuses for our oil & gas mineral interests. The Illinois Basin reportable segment includes currently operating mining complexes (a) Gibson County Coal, LLC's ("Gibson") mining complex, which includes the Gibson South mine, (b) the Warrior Coal, LLC ("Warrior") mining complex, (c) the River View Coal, LLC ("River View") mining complex and (d) the Hamilton mining complex. The Illinois Basin reportable segment also includes our currently operating Mt. Vernon Transfer Terminal, LLC ("Mt. Vernon") coal loading terminal in Indiana on the Ohio River. The Illinois Basin reportable segment also includes Mid-America Carbonates, LLC ("MAC") and other support services as well as non-operating mining complexes (a) Gibson North mine, which ceased production in the fourth quarter of 2019, (b) Webster County Coal, LLC's Dotiki mining complex, which ceased production in August 2019, (c) White County Coal, LLC's Pattiki mining complex, (d) the Hopkins County Coal, LLC mining complex, and (e) Sebree Mining, LLC's mining complex. The Appalachia reportable segment includes currently operating mining complexes (a) the Mettiki mining complex, (b) the Tunnel Ridge mining complex and (c) the MC Mining, LLC ("MC Mining") mining complex. The Mettiki mining complex includes Mettiki Coal (WV), LLC's Mountain View mine and Mettiki Coal, LLC's preparation plant. The Appalachia reportable segment also includes the Penn Ridge assets, which are primarily coal mineral interests. The Minerals reportable segment includes oil & gas mineral interests held by AR Midland and AllDale I & II and includes Alliance Minerals' equity interests in both AllDale III (Note 13 – Investments) and Cavalier Minerals. AR Midland acquired its mineral interest in the Wing Acquisition (Note 3 – Acquisitions). Other and Corporate includes marketing and administrative activities, Matrix Design Group, LLC and its subsidiaries ("Matrix Design"), Alliance Design Group, LLC ("Alliance Design") (collectively, Matrix Design and Alliance Design referred to as the "Matrix Group"), Alliance Coal's coal brokerage activity and Alliance Minerals' prior equity investment in Kodiak. In February 2019, Kodiak redeemed our equity investment (see Note 13 – Investments). In addition, Other and Corporate includes certain Alliance Resource Properties, LLC's land and coal mineral interest activities, Pontiki Coal, LLC's workers' compensation and pneumoconiosis liabilities, Wildcat Insurance, which assists the ARLP Partnership with its insurance requirements, and AROP Funding and Alliance Finance (both discussed in Note 8 – Long-Term Debt). In response to the impacts of the COVID-19 pandemic, we announced on March 30, 2020 a temporary cessation of coal production at our River View, Gibson, Hamilton and Warrior mining complexes in our Illinois Basin segment and on April 9, 2020 a temporary cessation of coal production at our MC Mining complex in our Appalachia segment. Underground production operations resumed in the second quarter of 2020 at each of our mining complexes. All of our seven mining complexes are now producing coal. However, several mines continue running at less than capacity due to a limited spot market in the United States and a seaborne market that continues to be sub-economic for United States production. Due to the ongoing and unforeseen impacts of the COVID-19 pandemic, on April 26, 2020, the employment of 116 employees of Gibson and 78 employees of the Hamilton mining complexes was terminated permanently. In addition to reduced production levels and employment adjustments, we took numerous actions in 2020 to optimize cash flows and preserve liquidity by reducing capital expenditures, working capital, costs and expenses, including adjusting our corporate support structure to better align to current operating levels. Reportable segment results are presented below. Illinois Other and Elimination Basin Appalachia Minerals Corporate (1) Consolidated (in thousands) Year Ended December 31, 2020 Revenues - Outside $ 770,051 $ 500,330 $ 43,141 $ 14,607 $ — $ 1,328,129 Revenues - Intercompany — — — 10,517 (10,517) — Total revenues (2) 770,051 500,330 43,141 25,124 (10,517) 1,328,129 Segment Adjusted EBITDA Expense (3) 520,324 319,730 4,106 18,543 (1,454) 861,249 Segment Adjusted EBITDA (4) 236,911 172,288 39,773 6,580 (9,063) 446,489 Total assets 1,018,916 448,567 613,916 477,469 (392,852) 2,166,016 Capital expenditures 48,648 70,960 — 1,493 — 121,101 Year Ended December 31, 2019 Revenues - Outside $ 1,219,618 $ 644,389 $ 53,036 $ 44,677 $ — $ 1,961,720 Revenues - Intercompany 16,690 — — 12,173 (28,863) — Total revenues (2) 1,236,308 644,389 53,036 56,850 (28,863) 1,961,720 Segment Adjusted EBITDA Expense (3) 756,423 423,623 7,811 36,845 (19,806) 1,204,896 Segment Adjusted EBITDA (4) 385,200 215,950 46,997 32,911 (9,057) 672,001 Total assets 1,373,516 500,027 643,213 541,261 (471,323) 2,586,694 Capital expenditures (5) 189,270 111,739 — 4,849 — 305,858 Year Ended December 31, 2018 Revenues - Outside $ 1,289,898 $ 643,898 $ — $ 69,061 $ — $ 2,002,857 Revenues - Intercompany 31,191 67 — 12,431 (43,689) — Total revenues (2) 1,321,089 643,965 — 81,492 (43,689) 2,002,857 Segment Adjusted EBITDA Expense (3) 796,370 398,243 — 52,321 (35,134) 1,211,800 Segment Adjusted EBITDA (4) 417,773 240,286 21,323 44,864 (8,555) 715,691 Total assets 1,380,912 440,518 161,312 589,010 (177,004) 2,394,748 Capital expenditures 166,468 64,037 — 2,975 — 233,480 (1) The elimination column represents the elimination of intercompany transactions and is primarily comprised of sales from the Matrix Group to our mining operations, coal sales and purchases between operations within different segments, sales of receivables to AROP Funding, financing between segments and insurance premiums paid to Wildcat Insurance. (2) Revenues included in the Other and Corporate column are primarily attributable to the outside and affiliate revenues at the Matrix Group and coal brokerage activities. In additions, Other and Corporate includes affiliate revenues for administrative and Wildcat Insurance services. (3) Segment Adjusted EBITDA Expense includes operating expenses, coal purchases and other income. Transportation expenses are excluded as transportation revenues are recognized in an amount equal to transportation expenses when title passes to the customer. The following is a reconciliation of consolidated Segment Adjusted EBITDA Expense to Operating expenses (excluding depreciation, depletion and amortization) Year Ended December 31, 2020 2019 2018 (in thousands) Segment Adjusted EBITDA Expense $ 861,249 $ 1,204,896 $ 1,211,800 Outside coal purchases — (23,357) (1,466) Other income (expense) (1,593) 561 (2,621) Operating expenses (excluding depreciation, depletion and amortization) $ 859,656 $ 1,182,100 $ 1,207,713 (4) Segment Adjusted EBITDA is defined as net income attributable to ARLP before net interest expense, income taxes, depreciation, depletion and amortization, general and administrative expense, settlement gain, asset and goodwill impairments and acquisition gain. Management therefore is able to focus solely on the evaluation of segment operating profitability as it relates to our revenues and operating expenses, which are primarily controlled by our segments. Consolidated Segment Adjusted EBITDA is reconciled to net income (loss) as follows: Year Ended December 31, 2020 2019 2018 (in thousands) Consolidated Segment Adjusted EBITDA $ 446,489 $ 672,001 $ 715,691 General and administrative (59,806) (72,997) (68,298) Depreciation, depletion and amortization (313,387) (309,075) (280,225) Settlement gain — — 80,000 Asset impairments (24,977) (15,190) (40,483) Goodwill impairment (132,026) — — Interest expense, net (45,478) (45,496) (40,059) Acquisition gain — 177,043 — Income tax (expense) benefit (35) 211 (22) Acquisition gain attributable to noncontrolling interest — (7,083) — Net income (loss) attributable to ARLP $ (129,220) $ 399,414 $ 366,604 Noncontrolling interest 169 7,512 866 Net income (loss) $ (129,051) $ 406,926 $ 367,470 . (5) Capital Expenditures shown exclude the AllDale Acquisition on January 3, 2019 and the Wing Acquisition on August 2, 2019 (Note 3 – Acquisitions).

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	12 Months Ended
SUBSEQUENT EVENTS	Dec. 31, 2020
SUBSEQUENT EVENTS
SUBSEQUENT EVENTS	25. SUBSEQUENT EVENTS Other than the event described in Note 8, there were no subsequent events.

SCHEDULE I CONSOLIDATED FINANCI

SCHEDULE I CONSOLIDATED FINANCIAL INFORMATION OF REGISTRANT	12 Months Ended
SCHEDULE I CONSOLIDATED FINANCIAL INFORMATION OF REGISTRANT	Dec. 31, 2020
CONDENSED FINANCIAL INFORMATION OF REGISTRANT
CONDENSED FINANCIAL INFORMATION OF REGISTRANT	SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF REGISTRANT ALLIANCE RESOURCE PARTNERS, L.P. CONDENSED BALANCE SHEETS (PARENT) DECEMBER 31, 2020 AND 2019 (In thousands, except unit data) December 31, 2020 2019 ASSETS CURRENT ASSETS: Cash and cash equivalents $ 2,174 $ 2,176 Total current assets 2,174 2,176 OTHER ASSETS: Investments in consolidated subsidiaries 1,146,491 1,329,406 Total other assets 1,146,491 1,329,406 TOTAL ASSETS $ 1,148,665 $ 1,331,582 LIABILITIES AND PARTNERS' CAPITAL CURRENT LIABILITIES: Accrued taxes other than income taxes $ 100 $ 100 Total current liabilities 100 100 Total liabilities 100 100 PARTNERS' CAPITAL: Limited Partners - Common Unitholders 127,195,219 and 126,915,597 units outstanding, respectively 1,148,565 1,331,482 TOTAL LIABILITIES AND PARTNERS' CAPITAL $ 1,148,665 $ 1,331,582 See accompanying notes. CONDENSED STATEMENTS OF OPERATIONS (PARENT) FOR THE YEARS ENDED DECEMBER 31, 2020, 2019 AND 2018 (In thousands, except unit and per unit data) Year Ended December 31, 2020 2019 2018 EXPENSES: General and administrative $ — $ 41 $ 30 Total operating expenses — 41 30 INCOME (LOSS) FROM OPERATIONS — (41) (30) Interest income 24 34 22 Equity in earnings of consolidated subsidiaries (129,244) 399,421 366,612 NET INCOME (LOSS) ATTRIBUTABLE TO ARLP $ (129,220) $ 399,414 $ 366,604 NET INCOME (LOSS) ATTRIBUTABLE TO ARLP GENERAL PARTNER $ — $ — $ 1,560 LIMITED PARTNERS $ (129,220) $ 399,414 $ 365,044 EARNINGS PER LIMITED PARTNER UNIT - BASIC AND DILUTED $ (1.02) $ 3.07 $ 2.74 WEIGHTED-AVERAGE NUMBER OF UNITS OUTSTANDING – BASIC AND DILUTED 127,164,659 128,116,670 130,758,169 See accompanying notes. CONDENSED STATEMENTS OF CASH FLOWS (PARENT) FOR THE YEARS ENDED DECEMBER 31, 2020, 2019 AND 2018 (In thousands) Year Ended December 31, 2020 2019 2018 CASH FLOWS FROM OPERATING ACTIVITIES: $ 51,751 $ 278,308 $ 275,924 CASH FLOWS FROM FINANCING ACTIVITIES: Distributions paid to Partners (51,753) (278,425) (275,902) Net cash used in financing activities (51,753) (278,425) (275,902) NET CHANGE IN CASH AND CASH EQUIVALENTS (2) (117) 22 CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD 2,176 2,293 2,271 CASH AND CASH EQUIVALENTS AT END OF PERIOD $ 2,174 $ 2,176 $ 2,293 See accompanying notes. NOTES TO FINANCIAL INFORMATION (PARENT) 1. BASIS OF PRESENTATION In these parent-company-only financial statements, our investment in consolidated subsidiaries is stated at cost plus equity in undistributed earnings of subsidiaries and reduced by distributions received from subsidiaries since the date of acquisition. These parent-company-only financial statements should be read in conjunction with our consolidated financial statements in "Item 8. Financial Statements and Supplementary Data" of this Annual Report on Form 10-K. 2. GUARANTEES As the parent of the Intermediate Partnership, we are a guarantor of both the Credit Agreement and Senior Notes discussed in "Item 8. Financial Statements and Supplementary Data—Note 8 – Long-Term Debt" of this Annual Report on Form 10-K. In addition to these guarantees, we have provided guarantees on surety indemnity agreements and financially guaranteed certain coal supply agreements. The duration of these guarantees varies and the maximum undiscounted potential future payment obligation related to these guarantees as of December 31, 2020 is not material. 3. CASH DISTRIBUTIONS RECEIVED We received distributions of $51.8 million, $278.4 million and $275.9 million from our consolidated subsidiaries during the years ended December 31, 2020, 2019, and 2018, respectively.

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	Dec. 31, 2020
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Presentation	Presentation The consolidated financial statements include the accounts and operations of the ARLP Partnership and present our financial position as of December 31, 2020 and 2019, and results of our operations, comprehensive income, cash flows and changes in partners' capital for each of the three years in the period ended December 31, 2020. All of our intercompany transactions and accounts have been eliminated.
Consolidation	Consolidation
Variable Interest Entity ("VIE")	Variable Interest Entity ("VIE") To determine a VIE's primary beneficiary, we perform a qualitative assessment to determine which party, if any, has the power to direct activities of the VIE and the obligation to absorb losses and/or receive its benefits. This assessment involves identifying the activities that most significantly impact the VIE's economic performance and determine whether it, or another party, has the power to direct those activities. When evaluating whether we are the primary beneficiary of a VIE, we perform a qualitative analysis that considers the design of the VIE, the nature of our involvement and the variable interests held by other parties. See Note 12 – Variable Interest Entities for further information.
Estimates	Estimates ● Impairment assessments of investments, property, plant and equipment, and goodwill; ● Asset retirement obligations; ● Pension valuation variables; ● Workers' compensation and pneumoconiosis valuation variables; ● Acquisition related purchase price allocations; ● Life of mine assumptions; ● Oil & gas reserve quantities and carrying amounts; and ● Determination of oil & gas revenue accruals These significant estimates and assumptions are discussed throughout these notes to the consolidated financial statements.
Fair Value Measurements	Fair Value Measurements We use the following fair value hierarchy, which prioritizes the inputs to valuation techniques used to measure fair value into three broad levels: ● Level 1 – Quoted prices for identical assets and liabilities in active markets that we have the ability to access at the measurement date. ● Level 2 – Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model derived valuations whose inputs are observable or whose significant value drivers are observable. ● Level 3 – Unobservable inputs for the asset or liability including situations where there is little, if any, market activity for the asset or liability. The fair value hierarchy gives the highest priority to quoted prices in active markets (Level 1) and the lowest priority to unobservable data (Level 3). In some cases, the inputs used to measure fair value might fall into different levels of the fair value hierarchy. The lowest level input that is significant to a fair value measurement determines the applicable level in the fair value hierarchy. Assessing the significance of a particular input to the fair value measurement requires judgment, considering factors specific to the asset or liability. Significant fair value measurements are used in our significant estimates and are discussed throughout these notes.
Cash and Cash Equivalents	Cash and Cash Equivalents
Cash Management	Cash Management
Inventories	Inventories
Business Combinations	Business Combinations
Goodwill	Goodwill
Property, Plant and Equipment	Property, Plant and Equipment
Mine Development Costs	Mine Development Costs
Leases	Leases lease obligations. Some of our arrangements within these agreements have both lease and non-lease components, which are generally accounted for separately. We have elected a practical expedient to account for lease and non-lease components as a single lease component for leases of buildings and office equipment. Our leases have approximate lease terms of one year to 20 years, some of which include automatic renewals up to ten years which are likely to be exercised, and some of which include options to terminate the lease within one year. We also hold numerous mineral reserve leases with both related parties as well as third parties, none of which are accounted for as an operating lease or as a finance lease. We review each agreement to determine if an arrangement within the agreement contains a lease at the inception of an arrangement. Once an arrangement is determined to contain either an operating or finance lease with a term greater than 12 months, we recognize a lease liability for the obligation to make lease payments and a right-of-use asset for the right to use the underlying asset for the lease term based on the present value of lease payments over the lease term. The lease term includes all noncancelable periods defined in the lease as well as periods covered by options to extend the lease that we are reasonably certain to exercise. As an implicit borrowing rate cannot be determined under most of our leases, we use our incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. Expenses related to leases determined to be operating leases will be recognized on a straight-line basis over the lease term including any reasonably assured renewal periods, while those determined to be finance leases will be recognized following a front-loaded expense profile in which interest and amortization are presented separately in the income statement. The determination of whether a lease is accounted for as a finance lease or an operating lease requires management to make estimates primarily about the fair value of the asset and its estimated economic useful life.
Long-Lived Asset Impairment	Long-Lived Asset Impairment
Oil & Gas Reserve Quantities and Carrying Amounts	Oil & Gas Reserve Quantities and Carrying Amounts Mineral interests in oil & gas properties are grouped using a reasonable aggregation of properties with a common geological structural feature or stratigraphic condition, which we may also refer to as a depletable group. Mineral interests in proved oil & gas properties are depleted based on the units-of-production method. Proved reserves are quantities of oil & gas that can be estimated with reasonable certainty to be recoverable in the future from a given date forward, from known reservoirs, under existing economic conditions, operating methods, and government regulations. Proved developed resources are the quantities expected to be recovered through our operators' existing wells with existing equipment, infrastructure and operating methods. We evaluate impairment of our mineral interests in proved properties whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. This evaluation is performed on a depletable group basis. We compare the undiscounted projected future cash flows expected in connection with a depletable group to its unamortized carrying amount to determine recoverability. When the carrying amount of a depletable group exceeds its estimated undiscounted future cash flows, the carrying amount is written down to its fair value, which is measured as the present value of the projected future cash flows of such properties. The factors used to determine fair value include estimates of proved reserves, future commodity prices, timing of future production, future expenditures, and a risk-adjusted discount rate. Our mineral interests in unproved properties are also assessed for impairment periodically on a depletable group basis when facts and circumstances indicate that the carrying value may not be recoverable. Impairment of individual unproved properties whose acquisition costs are relatively significant are assessed on a property-by-property basis, and an impairment loss is recognized to the extent the carrying value exceeds the estimated recoverable value for the property. Impairment of unproved properties whose acquisition costs are not individually significant are assessed on a group basis. Any amount of loss to be recognized and the amount of a valuation allowance needed to provide for impairment of those properties is determined by amortizing those properties in the aggregate on the basis of historical experience and other relevant information, such as the relative proportion of such properties on which proved reserves have been found in the past. The carrying value of unproved properties, including unleased mineral rights, are determined based on management's assessment of fair value using factors similar to those previously noted for proved properties, as well as geographic and geologic data. Upon the sale of a complete depletable group, the book value thereof, less proceeds or salvage value, are charged to income. Upon the sale or retirement of an aggregation of interests which make up less than a complete depletable group, the proceeds are credited to accumulated depreciation, depletion and amortization, unless doing so would significantly alter the depreciation, depletion and amortization rate of the depletable group, in which case a gain or loss would be recorded.
Intangibles	Intangibles Prepaid expenses and other assets Other long-term assets December 31, 2020 December 31, 2019 Accumulated Intangibles, Accumulated Intangibles, Original Cost Amortization Net Original Cost Amortization Net (in thousands) Non-compete agreements $ — $ — $ — $ 9,803 $ (9,440) $ 363 Customer contracts and other 10,623 (5,744) 4,879 32,371 (24,258) 8,113 Mining permits 1,500 (373) 1,127 1,500 (307) 1,193 Total $ 12,123 $ (6,117) $ 6,006 $ 43,674 $ (34,005) $ 9,669 Amortization expense attributable to intangible assets is estimated as follows: Year Ended December 31, (in thousands) 2021 $ 2,831 2022 1,600 2023 647 2024 66 2025 66 Thereafter 795
Investments	Investments Our investments and ownership interests in entities in which we do not have a controlling financial interest are accounted for under the equity method of accounting if we have the ability to exercise significant influence over the entity. Investments accounted for under the equity method are initially recorded at cost, and the difference between the basis of our investment and the underlying equity in the net assets of the joint venture at the investment date, if any, is amortized over the lives of the related assets that gave rise to the difference. As of December 31, 2020 and 2019, we held an equity method investment in AllDale III through our subsidiary, Alliance Minerals. Prior to the AllDale Acquisition, our equity method investments also included AllDale I & II, both held through Cavalier Minerals. AllDale III and AllDale I & II are collectively referred to as the "AllDale Partnerships." See Note 13 – Investments for further discussion of our equity method investment in AllDale III and Note 3 – Acquisitions for discussion of the AllDale Acquisition. We review our investments for impairment whenever events or changes in circumstances indicate a loss in the value of the investment may be other-than-temporary.
Advance Royalties, net	Advance Royalties Advance royalties December 31, 2020 2019 (in thousands) Advance royalties, affiliates (see Note 21 – Related-Party Transactions) $ 48,389 $ 41,216 Advance royalties, third-parties 12,570 12,685 Total advance royalties $ 60,959 $ 53,901
Asset Retirement Obligations	Asset Retirement Obligations
Pension Benefits	Pension Benefits The discount rate is determined for our pension benefit plan based on an approach specific to our plan. The year end discount rate is determined considering a yield curve comprised of high-quality corporate bonds and the timing of the expected benefit cash flows. The expected long-term rate of return on plan assets is determined based on broad equity and bond indices, the investment goals and objectives, the target investment allocation and on the average annual total return for each asset class. Unrecognized actuarial gains and losses and unrecognized prior service costs and credits are deferred and recorded in accumulated other comprehensive loss until amortized as a component of net periodic benefit cost. Unrecognized actuarial gains and losses in excess of 10% of the greater of the benefit obligation or the market-related value of plan assets are amortized over the participants' average remaining future years of service.
Workers' Compensation and Pneumoconiosis (Black Lung) Benefits	Workers Compensation and Pneumoconiosis (Black Lung) Benefits We provide income replacement and medical treatment for work-related traumatic injury claims as required by applicable state laws. Workers' compensation laws also compensate survivors of workers who suffer employment related deaths. Our liability for traumatic injury claims is the estimated present value of current workers' compensation benefits, based on our actuarial estimates. Our actuarial calculations are based on a blend of actuarial projection methods and numerous assumptions including claim development patterns, mortality, medical costs and interest rates. Our pneumoconiosis benefits liability is calculated using the service cost method based on the actuarial present value of the estimated pneumoconiosis obligation. Our actuarial calculations are based on numerous assumptions including claim development patterns, medical costs and mortality. Actuarial gains or losses are amortized over the remaining service period of active miners. See Note 20 – Accrued Workers' Compensation and Pneumoconiosis Benefits for more information on Workers' Compensation and Pneumoconiosis Benefits.
Revenue Recognition	Coal Revenue Recognition The estimated transaction price from each of our contracts is based on the total amount of consideration we expect to be entitled to under the contract. Included in the transaction price for certain coal supply contracts is the impact of variable consideration, including quality price adjustments, handling services, government imposition claims, per ton price fluctuations based on certain coal sales price indices and anticipated payments in lieu of shipments. We have constrained the expected value of variable consideration in our estimation of transaction price and only included this consideration to the extent that it is probable that a significant revenue reversal will not occur. The estimated transaction price for each contract is allocated to our performance obligations based on relative standalone selling prices determined at contract inception. Variable consideration is allocated to a specific part of the contract in many instances, such as if the variable consideration is based on production activities for coal delivered during a certain period or the outcome of a customer's ability to accept coal shipments over a certain period. Contract assets are recorded as trade receivables and reported separately in our consolidated balance sheet from other contract assets as title passes to the customer and our right to consideration becomes unconditional. Payments for coal shipments are typically due within two four weeks Oil & Gas Revenue Recognition among other factors, whether a well connects to a gathering or transmission line, quality and heat content of the product, and prevailing supply and demand conditions. We also periodically earn revenue from lease bonuses. We recognize lease bonus revenue when we execute a lease of our mineral interests to exploration and production companies. A lease agreement represents our contract with an operator, which is generally an exploration and production company. The contract will a) generally transfer the rights to any oil or gas discovered, b) grant us a right to a specified royalty interest from the operator, and c) require the operator to commence drilling and complete operations within a specified time period. Control of the minerals transfers to the operator when the lease agreement is executed. At the time we execute the lease agreement, we expect to receive the lease bonus payment within a reasonable time, though in no case more than one year, such that we do not adjust the expected amount of consideration for the effects of any significant financing component. As a non-operator, we have limited visibility into the timing of when new wells start producing. In addition, production statements may not be received for 30 to 90 days or more after the date production is delivered. As a result, we are required to estimate the amount of production delivered to the purchaser and the price that will be received for the sale of the product. The expected sales volumes and prices from our properties are estimated and recorded within the Trade receivables
Common Unit-Based Compensation	Common Unit-Based Compensation We utilize the Supplemental Executive Retirement Plan ("SERP") to provide deferred compensation benefits for certain officers and key employees. All allocations made to participants under the SERP are made in the form of "phantom" ARLP units and SERP distributions will be settled in the form of ARLP common units. The SERP is administered by the Compensation Committee. Our directors participate in the MGP Amended and Restated Deferred Compensation Plan for Directors ("Directors' Deferred Compensation Plan"). Pursuant to the Directors' Deferred Compensation Plan, for amounts deferred either automatically or at the election of the director, a notional account is established and credited with notional common units of ARLP, described in the Directors' Deferred Compensation Plan as "phantom" units. Distributions from the Directors' Deferred Compensation Plan will be settled in the form of ARLP common units. For both the SERP and Directors' Deferred Compensation Plan, when quarterly cash distributions are made with respect to ARLP common units, an amount equal to such quarterly distribution is credited to each participant's notional account as additional phantom units. All grants of phantom units under the SERP and Directors' Deferred Compensation Plan vest immediately. The fair value of restricted common unit grants under the LTIP, SERP and the Directors' Deferred Compensation Plan are determined on the grant date of the award and recognized as compensation expense on a pro rata basis for LTIP and SERP awards, as appropriate, over the requisite service period. Compensation expense is fully recognized on the grant date for quarterly distributions credited to SERP accounts and Directors' Deferred Compensation Plan awards. The corresponding liability is classified as equity and included in limited partners' capital in the consolidated financial statements (See Note 17 – Compensation Plans).
Income Taxes	Income Taxes
New Accounting Standards Issued and Adopted	New Accounting Standards Issued and Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments–Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"). ASU 2016-13 changes the impairment model for most financial assets and certain other instruments to require the use of a new forward-looking "expected loss" model that generally will result in earlier recognition of allowances for losses. The new standard provides for the use of a modified retrospective transition method that allows for a cumulative-effect adjustment to retained earnings upon adoption. The new standard also requires disclosure of significantly more information related to these items. We adopted ASU 2016-13 on January 1, 2020. Because of the credit profile of our customers, the fact that we do not have a history of credit losses on our financial instruments and the absences of any material expected losses, the adoption of ASU 2016-13 did not have any material impact on our consolidated financial statements.

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	Dec. 31, 2020
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Summary of intangible assets	December 31, 2020 December 31, 2019 Accumulated Intangibles, Accumulated Intangibles, Original Cost Amortization Net Original Cost Amortization Net (in thousands) Non-compete agreements $ — $ — $ — $ 9,803 $ (9,440) $ 363 Customer contracts and other 10,623 (5,744) 4,879 32,371 (24,258) 8,113 Mining permits 1,500 (373) 1,127 1,500 (307) 1,193 Total $ 12,123 $ (6,117) $ 6,006 $ 43,674 $ (34,005) $ 9,669
Schedule of estimated amortization expense attributable to intangible assets	Year Ended December 31, (in thousands) 2021 $ 2,831 2022 1,600 2023 647 2024 66 2025 66 Thereafter 795
Summary of advance royalties	December 31, 2020 2019 (in thousands) Advance royalties, affiliates (see Note 21 – Related-Party Transactions) $ 48,389 $ 41,216 Advance royalties, third-parties 12,570 12,685 Total advance royalties $ 60,959 $ 53,901

ACQUISITIONS (Tables)

ACQUISITIONS (Tables)	12 Months Ended
ACQUISITIONS (Tables)	Dec. 31, 2020
AllDale I and II
ACQUISITION
Schedule of total fair value of consideration transferred	As of January 3, 2019 (in thousands) Cash $ 176,205 Previously held investments 307,322 Total $ 483,527
Summary of fair value allocation of assets acquired and liabilities assumed	(in thousands) Cash and cash equivalents $ 900 Mineral interests in proved properties 184,032 Mineral interests in unproved properties 291,190 Receivables 9,326 Accounts payable (1,921) Net assets acquired $ 483,527
Schedule of revenue and earnings since acquisition date and pro forma condensed consolidated income statement	Year Ended December 31, 2019 (in thousands) Revenue $ 48,411 Net income 18,543 Year Ended December 31, 2018 (in thousands) Total revenues As reported $ 2,002,857 Pro forma 2,042,545 Net income As reported $ 367,470 Pro forma 358,741
Wing
ACQUISITION
Summary of fair value allocation of assets acquired and liabilities assumed	As Previously Reported Adjustments Final (in thousands) Mineral interests in proved properties $ 58,084 16,987 $ 75,071 Mineral interests in unproved properties 84,976 (17,275) 67,701 Receivables 1,867 288 2,155 Net assets acquired $ 144,927 $ 144,927
Schedule of revenue and earnings since acquisition date and pro forma condensed consolidated income statement	Year Ended December 31, 2019 (in thousands) Revenue $ 4,625 Net income 1,291 The following represents our actual and pro forma consolidated revenues and net income for the years ended December 31, 2019 and 2018. Pro forma revenues and net income assumes the mineral interests acquired in the Wing Acquisition had been included in our consolidated results since January 1, 2018. These pro forma amounts have been calculated after applying our accounting policies. Year Ended December 31, 2019 2018 (in thousands) Total revenues As reported $ 1,961,720 $ 2,002,857 Pro forma 1,966,291 2,008,559 Net income As reported $ 406,926 $ 367,470 Pro forma 411,217 372,810

INVENTORIES (Tables)

INVENTORIES (Tables)	12 Months Ended
INVENTORIES (Tables)	Dec. 31, 2020
INVENTORIES
Schedule of inventories	December 31, 2020 2019 (in thousands) Coal $ 19,756 $ 63,645 Supplies (net of reserve for obsolescence of $5,547 and $5,555, respectively) 36,651 37,660 Total inventories, net $ 56,407 $ 101,305

PROPERTY, PLANT AND EQUIPMENT (

PROPERTY, PLANT AND EQUIPMENT (Tables)	12 Months Ended
PROPERTY, PLANT AND EQUIPMENT (Tables)	Dec. 31, 2020
PROPERTY, PLANT AND EQUIPMENT
Schedule of property, plant and equipment	December 31, 2020 2019 (in thousands) Mining equipment and processing facilities $ 1,896,324 $ 1,937,642 Land and coal mineral rights 454,310 453,237 Oil & gas mineral interests (1) 616,904 618,282 Buildings, office equipment and improvements 279,938 304,111 Construction and mine development in progress 25,799 86,876 Mine development costs 280,815 283,860 Property, plant and equipment, at cost 3,554,090 3,684,008 Less accumulated depreciation, depletion and amortization (1,753,845) (1,675,022) Total property, plant and equipment, net $ 1,800,245 $ 2,008,986 (1) Oil & gas mineral interests acquired in the AllDale and Wing Acquisitions. See Note 3 – Acquisitions for more information.

LONG-TERM DEBT (Tables)

LONG-TERM DEBT (Tables)	12 Months Ended
LONG-TERM DEBT (Tables)	Dec. 31, 2020
LONG-TERM DEBT
Schedule of long-term debt	Unamortized Discount and Principal Debt Issuance Costs December 31, December 31, 2020 2019 2020 2019 (in thousands) Revolving credit facility $ 87,500 $ 255,000 $ (7,196) $ (3,050) Senior notes 400,000 400,000 (3,964) (4,879) Securitization facility 55,900 73,800 — — May 2019 equipment financing 4,956 8,199 — — November 2019 equipment financing 42,367 52,281 — — June 2020 equipment financing 13,057 — — — 603,780 789,280 (11,160) (7,929) Less current maturities (73,199) (13,157) — — Total long-term debt $ 530,581 $ 776,123 $ (11,160) $ (7,929)
Schedule of aggregate maturities of long-term debt	Year Ended December 31, (in thousands) 2021 $ 73,199 2022 16,071 2023 24,970 2024 89,540 2025 400,000 $ 603,780

LEASES (Tables)

LEASES (Tables)	12 Months Ended
LEASES (Tables)	Dec. 31, 2020
LEASES
Schedule of lease cost and other information	The components of lease expense were as follows: December 31, 2020 2019 (in thousands) Finance lease cost: $ Amortization of right-of-use assets $ 704 14,608 Interest on lease liabilities 377 2,085 Operating lease cost 3,873 9,169 Short-term lease cost 84 464 Variable lease cost 1,375 1,360 Total lease cost $ 6,413 $ 27,686 Supplemental cash flow information related to leases was as follows: December 31, 2020 2019 (in thousands) Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows for operating leases $ 3,870 9,124 Operating cash flows for finance leases $ 377 891 Financing cash flows for finance leases $ 8,368 46,725 Right-of-use assets obtained in exchange for lease obligations: Operating leases $ 278 25,593 Supplemental balance sheet information related to leases was as follows: December 31, 2020 2019 (in thousands) Finance leases: Property and equipment finance lease assets, gross $ 5,485 $ 30,610 Accumulated depreciation (3,867) (20,564) Property and equipment finance lease assets, net $ 1,618 $ 10,046
Summary of supplemental lease information	December 31, 2020 2019 Weighted average remaining lease term Operating leases 13.4 years 13.1 years Finance leases 3.9 years 1.6 years Weighted average discount rate Operating leases 6.0 % 6.0 % Finance leases 8.0 % 6.0 %
Schedule of maturities of operating lease liabilities	Operating leases Finance leases (in thousands) 2021 $ 2,346 $ 912 2022 2,245 912 2023 2,061 139 2024 1,841 139 2025 1,527 139 Thereafter 11,838 280 Total lease payments 21,858 2,521 Less imputed interest (6,926) (297) Total $ 14,932 $ 2,224
Schedule of maturities of finance lease liabilities	Operating leases Finance leases (in thousands) 2021 $ 2,346 $ 912 2022 2,245 912 2023 2,061 139 2024 1,841 139 2025 1,527 139 Thereafter 11,838 280 Total lease payments 21,858 2,521 Less imputed interest (6,926) (297) Total $ 14,932 $ 2,224

FAIR VALUE MEASUREMENTS (Tables

FAIR VALUE MEASUREMENTS (Tables)	12 Months Ended
FAIR VALUE MEASUREMENTS (Tables)	Dec. 31, 2020
FAIR VALUE MEASUREMENTS
Summary of fair value measurements within the hierarchy	December 31, 2020 December 31, 2019 Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 (in thousands) Long-term debt $ — $ 518,317 $ — $ — $ 736,206 $ — Total $ — $ 518,317 $ — $ — $ 736,206 $ —

PARTNERS' CAPITAL (Tables)

PARTNERS' CAPITAL (Tables)	12 Months Ended
PARTNERS' CAPITAL (Tables)	Dec. 31, 2020
PARTNERS' CAPITAL
Summary of quarterly per unit distribution paid	Year Ended December 31, 2020 2019 2018 First Quarter $ 0.400 $ 0.530 $ 0.510 Second Quarter $ — $ 0.535 $ 0.515 Third Quarter $ — $ 0.540 $ 0.520 Fourth Quarter $ — $ 0.540 $ 0.525

VARIABLE INTEREST ENTITIES (Tab

VARIABLE INTEREST ENTITIES (Tables)	12 Months Ended
VARIABLE INTEREST ENTITIES (Tables)	Dec. 31, 2020
VARIABLE INTEREST ENTITIES
Schedule of distributions	Alliance Bluegrass Minerals Minerals (in thousands) Contributions $ 143,112 $ 5,963 Distributions 89,380 3,723

INVESTMENTS (Tables)

INVESTMENTS (Tables)	12 Months Ended
INVESTMENTS (Tables)	Dec. 31, 2020
INVESTMENTS
Schedule of changes in equity method investment	Year Ended December 31, 2020 2019 2018 (in thousands) Beginning balance $ 28,529 $ 28,974 $ 14,182 Contributions — — 15,600 Equity method investment income 907 2,203 547 Distributions received (1,895) (2,648) (1,355) Other (273) — — Ending balance $ 27,268 $ 28,529 $ 28,974

REVENUE FROM CONTRACTS WITH C_2

REVENUE FROM CONTRACTS WITH CUSTOMERS (Tables)	12 Months Ended
REVENUE FROM CONTRACTS WITH CUSTOMERS (Tables)	Dec. 31, 2020
REVENUE FROM CONTRACTS WITH CUSTOMERS
Schedule of disaggregation of revenues by type	Illinois Other and Basin Appalachia Minerals Corporate Elimination Consolidated (in thousands) Year Ended December 31, 2020 Coal sales $ 755,208 $ 477,064 $ — $ — $ — $ 1,232,272 Oil & gas royalties — — 42,912 — — 42,912 Transportation revenues 12,817 8,312 — — — 21,129 Other revenues 2,026 14,954 229 25,124 (10,517) 31,816 Total revenues $ 770,051 $ 500,330 $ 43,141 $ 25,124 $ (10,517) $ 1,328,129 Year Ended December 31, 2019 Coal sales $ 1,128,588 $ 628,406 $ — $ 22,138 $ (16,690) $ 1,762,442 Oil & gas royalties — — 51,735 — — 51,735 Transportation revenues 94,686 4,817 — — — 99,503 Other revenues 13,034 11,166 1,301 34,712 (12,173) 48,040 Total revenues $ 1,236,308 $ 644,389 $ 53,036 $ 56,850 $ (28,863) $ 1,961,720 Year Ended December 31, 2018 Coal sales $ 1,197,143 $ 635,530 $ — $ 43,393 $ (31,258) $ 1,844,808 Transportation revenues 106,947 5,435 — 3 — 112,385 Other revenues 16,999 3,000 — 38,096 (12,431) 45,664 Total revenues $ 1,321,089 $ 643,965 $ — $ 81,492 $ (43,689) $ 2,002,857
Schedule of current coal supply contracts allocated to performance obligations that are unsatisfied or partially unsatisfied	2024 and 2021 2022 2023 Thereafter Total (in thousands) Illinois Basin coal revenues $ 653,208 $ 253,654 $ 187,570 $ 140,750 $ 1,235,182 Appalachia coal revenues 318,984 95,471 — — 414,455 Total coal revenues (1) $ 972,192 $ 349,125 $ 187,570 $ 140,750 $ 1,649,637 (1) Coal revenues generally consists of consolidated revenues excluding our Minerals segment.

EARNINGS PER LIMITED PARTNER _2

EARNINGS PER LIMITED PARTNER UNIT (Tables)	12 Months Ended
EARNINGS PER LIMITED PARTNER UNIT (Tables)	Dec. 31, 2020
EARNINGS PER LIMITED PARTNER UNIT
Reconciliation of net income and EPU calculations	Year Ended December 31, 2020 2019 2018 (in thousands, except per unit data) Net income (loss) attributable to ARLP $ (129,220) $ 399,414 $ 366,604 Adjustment: General partner's equity ownership (1) — — (1,560) Limited partners' interest in net income (loss) attributable to ARLP (129,220) 399,414 365,044 Less: Distributions to participating securities — (4,254) (5,114) Undistributed earnings attributable to participating securities — (2,237) (1,641) Net income (loss) attributable to ARLP available to limited partners $ (129,220) $ 392,923 $ 358,289 Weighted-average limited partner units outstanding – basic and diluted 127,165 128,117 130,758 Earnings per limited partner unit - basic and diluted (2) $ (1.02) $ 3.07 $ 2.74 (1) Amounts presented for periods subsequent to the first quarter of 2018 reflect the impact of the Simplification Transactions, which ended net income allocations and quarterly cash distributions to MGP after May 31, 2018. Prior to the Simplification Transactions, MGP maintained a 1.0001% general partner interest in the Intermediate Partnership and a 0.001% managing member interest in Alliance Coal and thus received quarterly distributions and income and loss allocations during this time period. (2) Diluted EPU gives effect to all potentially dilutive common units outstanding during the period using the treasury stock method. Diluted EPU excludes all potentially dilutive units calculated under the treasury stock method if their effect is anti-dilutive. For the years ended December 31, 2020, 2019 and 2018, the combined total of LTIP, SERP and Directors' Deferred Compensation Plan units of 773,664 , 1,284,013 and 1,658,908 , respectively, were considered anti-dilutive under the treasury stock method.

EMPLOYEE BENEFIT PLANS (Tables)

EMPLOYEE BENEFIT PLANS (Tables) - Defined benefit pension plan	12 Months Ended
	Dec. 31, 2020
Employee Benefit Plans
Summary of benefit plans for employees	December 31, 2020 2019 (dollars in thousands) Change in benefit obligations: Benefit obligations at beginning of year $ 136,425 $ 118,958 Interest cost 4,185 4,864 Actuarial loss 12,396 17,084 Benefits paid (5,072) (4,481) Benefit obligations at end of year 147,934 136,425 Change in plan assets: Fair value of plan assets at beginning of year 91,567 75,823 Employer contribution 1,739 5,559 Actual return on plan assets 12,735 14,666 Benefits paid (5,072) (4,481) Fair value of plan assets at end of year 100,969 91,567 Funded status at the end of year $ (46,965) $ (44,858) Amounts recognized in balance sheet: Non-current liability $ (46,965) $ (44,858) Amounts recognized in accumulated other comprehensive income consists of: Prior service cost $ (754) $ (940) Net actuarial loss (46,519) (45,125) $ (47,273) $ (46,065) Weighted-average assumption to determine benefit obligations as of December 31, Discount rate 2.37% 3.15% Weighted-average assumptions used to determine net periodic benefit cost for the year ended December 31, Discount rate 3.15% 4.17% Expected return on plan assets 6.50% 6.50%
Schedule of long-term rate of return assumptions	Asset allocation As of December 31, 2020 assumption Equity securities 62% Fixed income securities 33% Real estate 5% 100%
Components of net periodic benefit cost	Year Ended December 31, 2020 2019 2018 (in thousands) Components of net periodic benefit cost: Interest cost $ 4,185 $ 4,864 $ 4,462 Expected return on plan assets (5,861) (4,932) (5,784) Amortization of prior service cost 186 186 186 Amortization of net loss 4,128 3,922 3,608 Net periodic benefit cost (1) $ 2,638 $ 4,040 $ 2,472 (1) Nonservice components of net periodic benefit cost are included in the Other income (expense) line item within our consolidated statements of income.
Schedule of other changes in plan assets and benefit obligation recognized in accumulated other comprehensive income	Year Ended December 31, 2020 2019 (in thousands) Other changes in plan assets and benefit obligation recognized in accumulated other comprehensive loss: Net actuarial loss $ (5,522) $ (7,350) Reversal of amortization item: Prior service cost 186 186 Net actuarial loss 4,128 3,922 Total recognized in accumulated other comprehensive loss (1,208) (3,242) Net periodic benefit cost (2,638) (4,040) Total recognized in net periodic benefit cost and accumulated other comprehensive loss $ (3,846) $ (7,282)
Schedule of estimated future benefit payments	Year Ended December 31, (in thousands) 2021 $ 5,629 2022 5,954 2023 6,269 2024 6,488 2025 6,620 2026-2030 34,674 $ 65,634
Schedule of asset allocation guidelines, actual asset allocations and fair value of Pension Plan assets	Percentage of Total Portfolio Minimum Target Maximum Equity securities 45% 62% 80% Fixed income securities 10% 33% 55% Real estate 0% 5% 10% December 31, 2020 2019 (in thousands) Cash and cash equivalents (a) $ 3,888 $ 2,958 Commingled investment funds measured at net asset value (b): Equities - Global 17,549 10,028 Equities - United States 31,835 26,812 Equities - United States futures (2,616) — Equities - International developed markets 8,920 10,528 Equities - International developed markets futures (4,921) — Equities - International emerging markets 6,600 8,410 Equities - International emerging markets futures (975) — Fixed income - Investment grade 25,703 26,186 Fixed income - High yield 10,056 — Fixed income - Emerging markets 2,664 — Fixed income - Futures (1,265) — Real estate 3,531 4,355 Other — 2,290 Total $ 100,969 $ 91,567 (a) Cash and cash equivalents represents a Level 1 fair value measurement. See Note 2 – Summary of Significant Accounting Policies – Fair Value Measurements for more information regarding the definitions of fair value hierarchy levels. (b) Investments measured at fair value using the net asset value per share (or its equivalent) have not been classified within the fair value hierarchy. The fair values of all commingled investment funds are determined based on the net asset values per unit of each of the funds. The net asset values per unit represent the aggregate value of the fund's assets at fair value less liabilities, divided by the number of units outstanding.

COMMON UNIT-BASED COMPENSATIO_2

COMMON UNIT-BASED COMPENSATION PLANS (Tables)	12 Months Ended
COMMON UNIT-BASED COMPENSATION PLANS (Tables)	Dec. 31, 2020
ARLP LTIP
COMMON UNIT-BASED COMPENSATION PLANS
Summary of activity in share-based plans	Number of units Weighted average grant date fair value per unit Intrinsic value (in thousands) Non-vested grants at January 1, 2018 1,694,026 $ 19.62 $ 33,372 Granted 511,305 20.40 Vested (1) (331,502) 34.61 Forfeited (45,749) 17.40 Non-vested grants at December 31, 2018 1,828,080 17.18 31,699 Granted 682,155 18.63 Vested (1) (885,381) 12.38 Forfeited (21,476) 20.84 Non-vested grants at December 31, 2019 1,603,378 20.39 17,349 Granted (2) 1,430,489 5.02 Vested (3) (919,524) 21.70 Grants canceled (4) (675,302) 18.62 Forfeited (8,552) 20.16 Non-vested grants at December 31, 2020 1,430,489 5.02 6,409 (1) During the years ended December 31, 2019 and 2018, we issued 596,650 and 191,858 , respectively, unrestricted common units to LTIP participants. The remaining vested units were settled in cash to satisfy tax withholding obligations of the LTIP participants. (2) In December 2020, we modified the vesting requirements for certain restricted units that we granted in February 2020 which were determined to be improbable of vesting under the original vesting requirements (the "2020 Grants"). The new vesting requirements make it probable the modified restricted units will vest. Also in December 2020, an additional 578,114 restricted units under these modified vesting requirements were granted. The grant date fair value reflects the modification date fair value for those awards that were modified. (3) In February 2020, we issued 279,622 unrestricted common units to LTIP participants as a result of satisfying the vesting requirements for 424,486 restricted units that were granted in 2017. The remaining vested units were settled in cash to satisfy tax withholding obligations of the LTIP participants. In December 2020, we accelerated the vesting requirements for 495,038 restricted units that were granted in 2018 (the "2018 Grants") and settled these restricted units in cash. (4) In December 2020, 675,302 restricted units that were granted in 2019 (the "2019 Grants") were canceled since it was determined that the vesting requirements for these restricted units were not probable of being satisfied.
SERP and Directors' Compensation Plans
COMMON UNIT-BASED COMPENSATION PLANS
Summary of activity in share-based plans	Number of units Weighted average grant date fair value per unit Intrinsic value (in thousands) Phantom units outstanding as of January 1, 2018 561,784 $ 28.64 $ 11,067 Granted 84,417 18.78 Issued (1) (10,364) 27.92 Phantom units outstanding as of December 31, 2018 635,837 27.34 11,025 Granted 111,012 14.50 Issued (1) (115,484) 25.20 Phantom units outstanding as of December 31, 2019 631,365 25.48 6,831 Granted 129,265 5.25 Phantom units outstanding as of December 31, 2020 760,630 22.04 3,408 (1) During the years ended December 31, 2019 and 2018, we issued ARLP common units of 115,484 and 7,181 , respectively, to participants under the SERP and Directors' Deferred Compensation Plan . Units issued in 2018 were net of units settled in cash to satisfy tax withholding obligations.

SUPPLEMENTAL CASH FLOW INFORM_2

SUPPLEMENTAL CASH FLOW INFORMATION (Tables)	12 Months Ended
SUPPLEMENTAL CASH FLOW INFORMATION (Tables)	Dec. 31, 2020
SUPPLEMENTAL CASH FLOW INFORMATION
Schedule of supplemental cash flow information	Year Ended December 31, 2020 2019 2018 (in thousands) Cash Paid For: Interest $ 44,226 $ 43,093 $ 38,450 Income taxes $ 12 $ — $ 34 Non-Cash Activity: Accounts payable for purchase of property, plant and equipment $ 5,731 $ 14,504 $ 14,585 Right-of-use assets acquired by operating lease $ 278 25,593 — Market value of common units issued under deferred compensation plans before tax withholding requirements $ 3,837 $ 17,415 $ 6,142

ASSET RETIREMENT OBLIGATIONS (T

ASSET RETIREMENT OBLIGATIONS (Tables)	12 Months Ended
ASSET RETIREMENT OBLIGATIONS (Tables)	Dec. 31, 2020
ASSET RETIREMENT OBLIGATIONS
Schedule of activity affecting the asset retirement and mine closing liability	Year Ended December 31, 2020 2019 (in thousands) Beginning balance $ 137,514 $ 137,114 Accretion expense 4,033 4,087 Payments (1,769) (2,948) Allocation of liability associated with acquisitions, mine development and change in assumptions (11,880) (739) Ending balance $ 127,898 $ 137,514
Schedule of estimated payments of asset retirement obligations	Year Ended December 31, (in thousands) 2021 $ 6,411 2022 2,723 2023 2,570 2024 3,317 2025 4,601 Thereafter 210,330 Aggregate undiscounted asset retirement obligations 229,952 Effect of discounting (102,054) Total asset retirement obligations 127,898 Less: current portion (6,411) Non-current asset retirement obligations $ 121,487

ACCRUED WORKERS' COMPENSATION_2

ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS (Tables)	12 Months Ended
	Dec. 31, 2020
Accrued Workers Compensation And Pneumoconiosis Benefits
Reconciliation of changes in workers' compensation liability	December 31, 2020 2019 (in thousands) Beginning balance $ 53,384 $ 49,539 Accruals increase 5,146 7,162 Payments (8,482) (11,320) Interest accretion 1,278 1,606 Valuation loss 3,413 6,397 Ending balance $ 54,739 $ 53,384
Reconciliation of changes in pneumoconiosis benefit obligation	December 31, 2020 2019 (in thousands) Benefit obligations at beginning of year $ 97,683 $ 72,095 Service cost 3,526 2,593 Interest cost 2,998 3,044 Actuarial (gain) loss 7,787 23,298 Benefits and expenses paid (3,498) (3,347) Benefit obligations at end of year $ 108,496 $ 97,683
Components of pneumoconiosis and workers' compensation expense	Year Ended December 31, 2020 2019 2018 (in thousands) Black lung benefits: Service cost $ 3,526 $ 2,593 $ 2,525 Interest cost (1) 2,998 3,044 2,542 Net amortization (1) (686) (4,582) 2 Total pneumoconiosis expense 5,838 1,055 5,069 Workers' compensation expense 12,305 17,541 11,270 Net periodic benefit cost $ 18,143 $ 18,596 $ 16,339 ________________________________________ (1) Interest cost and net amortization is included in the Other income (expense) line item within our consolidated statements of income ( see Note 2 – Summary of Significant Accounting Policies).
Pneumoconiosis benefits
Accrued Workers Compensation And Pneumoconiosis Benefits
Reconciliation of changes in the pneumoconiosis benefit obligation recognized in AOCI	Year Ended December 31, 2020 2019 2018 (in thousands) Net actuarial gain (loss) $ (7,787) $ (23,298) $ 4,599 Reversal of amortization item: Net actuarial (gain) loss (686) (4,582) 2 Total recognized in accumulated other comprehensive loss $ (8,473) $ (27,880) $ 4,601
Schedule of amount recognized in accumulated other comprehensive income	Year Ended December 31, 2020 2019 2018 (in thousands) Amount recognized in accumulated other comprehensive loss consists of: Net actuarial loss $ 40,399 $ 31,927 $ 4,047
Summary of information about amounts recognized in the consolidated balance sheets	December 31, 2020 2019 (in thousands) Workers’ compensation claims $ 54,739 $ 53,384 Pneumoconiosis benefit claims 108,496 97,683 Total obligations 163,235 151,067 Less current portion (10,646) (11,175) Non-current obligations $ 152,589 $ 139,892

RELATED-PARTY TRANSACTIONS (Tab

RELATED-PARTY TRANSACTIONS (Tables)	12 Months Ended
RELATED-PARTY TRANSACTIONS (Tables)	Dec. 31, 2020
RELATED-PARTY TRANSACTIONS
Summary of advanced royalties outstanding	WKY CoalPlay Towhead Webster Henderson WKY SGP/Craft Foundations Coal Coal Coal CoalPlay Henderson Henderson Tunnel & Union Webster Henderson & Union Ridge Counties, KY County, KY County, KY Counties, KY Total Acquired Acquired Acquired Acquired Acquired 2005 December 2014 December 2014 December 2014 February 2015 (in thousands) As of January 1, 2018 $ 3,000 $ 10,684 $ 5,356 $ 7,566 $ 6,387 $ 32,993 Payments — 3,597 2,570 2,520 2,131 10,818 Recoupment (3,000) (204) (31) — (36) (3,271) Unrecoupable — — (7,895) — — (7,895) As of December 31, 2018 — 14,077 — 10,086 8,482 32,645 Payments 4,500 3,597 2,568 2,521 2,131 15,317 Recoupment (3,000) (1,071) — — (107) (4,178) Unrecoupable — — (2,568) — — (2,568) As of December 31, 2019 1,500 16,603 — 12,607 10,506 41,216 Payments 3,000 3,597 2,568 2,522 2,132 13,819 Recoupment (3,000) (1,022) — — (56) (4,078) Unrecoupable — — (2,568) — — (2,568) As of December 31, 2020 $ 1,500 $ 19,178 $ — $ 15,129 $ 12,582 $ 48,389

CONCENTRATION OF CREDIT RISK _2

CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS (Tables)	12 Months Ended
CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS (Tables)	Dec. 31, 2020
CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS
Schedule of total revenues from major customers	Year Ended December 31, Segment 2020 2019 2018 (in thousands) Customer A Illinois Basin $ 197,379 $ 228,500 $ 219,115 Customer B Appalachia — 213,319 — Customer C Illinois Basin 157,271 — — Customer D Illinois Basin/Appalachia 137,785 — —

SEGMENT INFORMATION (Tables)

SEGMENT INFORMATION (Tables)	12 Months Ended
SEGMENT INFORMATION (Tables)	Dec. 31, 2020
SEGMENT INFORMATION
Schedule of reportable segment results	Illinois Other and Elimination Basin Appalachia Minerals Corporate (1) Consolidated (in thousands) Year Ended December 31, 2020 Revenues - Outside $ 770,051 $ 500,330 $ 43,141 $ 14,607 $ — $ 1,328,129 Revenues - Intercompany — — — 10,517 (10,517) — Total revenues (2) 770,051 500,330 43,141 25,124 (10,517) 1,328,129 Segment Adjusted EBITDA Expense (3) 520,324 319,730 4,106 18,543 (1,454) 861,249 Segment Adjusted EBITDA (4) 236,911 172,288 39,773 6,580 (9,063) 446,489 Total assets 1,018,916 448,567 613,916 477,469 (392,852) 2,166,016 Capital expenditures 48,648 70,960 — 1,493 — 121,101 Year Ended December 31, 2019 Revenues - Outside $ 1,219,618 $ 644,389 $ 53,036 $ 44,677 $ — $ 1,961,720 Revenues - Intercompany 16,690 — — 12,173 (28,863) — Total revenues (2) 1,236,308 644,389 53,036 56,850 (28,863) 1,961,720 Segment Adjusted EBITDA Expense (3) 756,423 423,623 7,811 36,845 (19,806) 1,204,896 Segment Adjusted EBITDA (4) 385,200 215,950 46,997 32,911 (9,057) 672,001 Total assets 1,373,516 500,027 643,213 541,261 (471,323) 2,586,694 Capital expenditures (5) 189,270 111,739 — 4,849 — 305,858 Year Ended December 31, 2018 Revenues - Outside $ 1,289,898 $ 643,898 $ — $ 69,061 $ — $ 2,002,857 Revenues - Intercompany 31,191 67 — 12,431 (43,689) — Total revenues (2) 1,321,089 643,965 — 81,492 (43,689) 2,002,857 Segment Adjusted EBITDA Expense (3) 796,370 398,243 — 52,321 (35,134) 1,211,800 Segment Adjusted EBITDA (4) 417,773 240,286 21,323 44,864 (8,555) 715,691 Total assets 1,380,912 440,518 161,312 589,010 (177,004) 2,394,748 Capital expenditures 166,468 64,037 — 2,975 — 233,480 (1) The elimination column represents the elimination of intercompany transactions and is primarily comprised of sales from the Matrix Group to our mining operations, coal sales and purchases between operations within different segments, sales of receivables to AROP Funding, financing between segments and insurance premiums paid to Wildcat Insurance. (2) Revenues included in the Other and Corporate column are primarily attributable to the outside and affiliate revenues at the Matrix Group and coal brokerage activities. In additions, Other and Corporate includes affiliate revenues for administrative and Wildcat Insurance services. (3) Segment Adjusted EBITDA Expense includes operating expenses, coal purchases and other income. Transportation expenses are excluded as transportation revenues are recognized in an amount equal to transportation expenses when title passes to the customer. The following is a reconciliation of consolidated Segment Adjusted EBITDA Expense to Operating expenses (excluding depreciation, depletion and amortization) Year Ended December 31, 2020 2019 2018 (in thousands) Segment Adjusted EBITDA Expense $ 861,249 $ 1,204,896 $ 1,211,800 Outside coal purchases — (23,357) (1,466) Other income (expense) (1,593) 561 (2,621) Operating expenses (excluding depreciation, depletion and amortization) $ 859,656 $ 1,182,100 $ 1,207,713 (4) Segment Adjusted EBITDA is defined as net income attributable to ARLP before net interest expense, income taxes, depreciation, depletion and amortization, general and administrative expense, settlement gain, asset and goodwill impairments and acquisition gain. Management therefore is able to focus solely on the evaluation of segment operating profitability as it relates to our revenues and operating expenses, which are primarily controlled by our segments. Consolidated Segment Adjusted EBITDA is reconciled to net income (loss) as follows: Year Ended December 31, 2020 2019 2018 (in thousands) Consolidated Segment Adjusted EBITDA $ 446,489 $ 672,001 $ 715,691 General and administrative (59,806) (72,997) (68,298) Depreciation, depletion and amortization (313,387) (309,075) (280,225) Settlement gain — — 80,000 Asset impairments (24,977) (15,190) (40,483) Goodwill impairment (132,026) — — Interest expense, net (45,478) (45,496) (40,059) Acquisition gain — 177,043 — Income tax (expense) benefit (35) 211 (22) Acquisition gain attributable to noncontrolling interest — (7,083) — Net income (loss) attributable to ARLP $ (129,220) $ 399,414 $ 366,604 Noncontrolling interest 169 7,512 866 Net income (loss) $ (129,051) $ 406,926 $ 367,470 . (5) Capital Expenditures shown exclude the AllDale Acquisition on January 3, 2019 and the Wing Acquisition on August 2, 2019 (Note 3 – Acquisitions).
Reconciliation of consolidated Segment Adjusted EBITDA Expense to operating expenses (excluding depreciation, depletion and amortization)	Year Ended December 31, 2020 2019 2018 (in thousands) Segment Adjusted EBITDA Expense $ 861,249 $ 1,204,896 $ 1,211,800 Outside coal purchases — (23,357) (1,466) Other income (expense) (1,593) 561 (2,621) Operating expenses (excluding depreciation, depletion and amortization) $ 859,656 $ 1,182,100 $ 1,207,713
Reconciliation of consolidated Segment Adjusted EBITDA to net income	Year Ended December 31, 2020 2019 2018 (in thousands) Consolidated Segment Adjusted EBITDA $ 446,489 $ 672,001 $ 715,691 General and administrative (59,806) (72,997) (68,298) Depreciation, depletion and amortization (313,387) (309,075) (280,225) Settlement gain — — 80,000 Asset impairments (24,977) (15,190) (40,483) Goodwill impairment (132,026) — — Interest expense, net (45,478) (45,496) (40,059) Acquisition gain — 177,043 — Income tax (expense) benefit (35) 211 (22) Acquisition gain attributable to noncontrolling interest — (7,083) — Net income (loss) attributable to ARLP $ (129,220) $ 399,414 $ 366,604 Noncontrolling interest 169 7,512 866 Net income (loss) $ (129,051) $ 406,926 $ 367,470

ORGANIZATION AND PRESENTATION (

ORGANIZATION AND PRESENTATION (Details)	Aug. 02, 2019a	Feb. 08, 2019a	Jan. 03, 2019a	May 31, 2018shares	Dec. 31, 2020
AllDale I and II
Ownership interests
Royalty acres, net \| a			43,000
Wing
Ownership interests
Royalty acres controlled \| a		55,700
Royalty acres, net \| a	9,000
Royalty acres, gross \| a	400,000
SGP \| AHGP
Ownership interests
Ownership percentage by limited partners					34.48%
Simplification Transactions \| SGP
Ownership interests
Common units issued in exchange \| shares				29,188,997
Issuance of units to Owners of SGP in Simplification Transactions (in units) \| shares				1,322,388
Simplification Transactions \| AHGP
Ownership interests
Units issued in exchange (in units) \| shares				1.4782
Simplification Transactions \| Intermediate Partnership
Ownership interests
Ownership percentage by general partners				1.0001%
Simplification Transactions \| Alliance Coal
Ownership interests
Ownership percentage by general partners				0.001%
Simplification Transactions \| AHGP
Ownership interests
Approved written consent percent				68.00%
Units issued in exchange (in units) \| shares				1.4782
Simplification Transactions \| AHGP \| Intermediate Partnership
Ownership interests
Ownership percentage by general partners				1.0001%
Simplification Transactions \| AHGP \| Alliance Coal
Ownership interests
Ownership percentage by general partners				0.001%
Simplification Transactions \| SGP \| Intermediate Partnership
Ownership interests
Ownership percentage by general partners				1.0001%
Simplification Transactions \| SGP \| Alliance Coal
Ownership interests
Ownership percentage by general partners				0.001%

SUMMARY OF SIGNIFICANT ACCOUN_4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill
Impairments of goodwill	$ 132,026	$ 0	$ 0

SUMMARY OF SIGNIFICANT ACCOUN_5

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Tangible Assets (Details)	12 Months Ended
	Dec. 31, 2020
Preparation plants, processing facilities and mineral rights \| Minimum
Property, Plant and Equipment
Estimated useful life	1 year
Preparation plants, processing facilities and mineral rights \| Maximum
Property, Plant and Equipment
Estimated useful life	22 years
Other plant and equipment and mining equipment \| Minimum
Property, Plant and Equipment
Estimated useful life	1 year
Other plant and equipment and mining equipment \| Maximum
Property, Plant and Equipment
Estimated useful life	22 years
Buildings, office equipment and improvements \| Minimum
Property, Plant and Equipment
Estimated useful life	1 year
Buildings, office equipment and improvements \| Maximum
Property, Plant and Equipment
Estimated useful life	22 years

SUMMARY OF SIGNIFICANT ACCOUN_6

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Leases (Details)	12 Months Ended
	Dec. 31, 2020
Lease renewal term	10 years
Lease termination option term	1 year
Minimum
Lease term	1 year
Maximum
Lease term	20 years

SUMMARY OF SIGNIFICANT ACCOUN_7

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Intangible Assets
Amortization expense attributable to intangible assets	$ 4,900	$ 9,100	$ 6,900
Original Cost	12,123	43,674
Accumulated Amortization	(6,117)	(34,005)
Intangibles, Net	6,006	9,669
Estimated amortization expense attributable to intangible assets
2021	2,831
2022	1,600
2023	647
2024	66
2025	66
Thereafter	795
Noncompete agreement
Intangible Assets
Original Cost		9,803
Accumulated Amortization		(9,440)
Intangibles, Net		363
Customer contracts, net
Intangible Assets
Original Cost	10,623	32,371
Accumulated Amortization	(5,744)	(24,258)
Intangibles, Net	4,879	8,113
Permits
Intangible Assets
Original Cost	1,500	1,500
Accumulated Amortization	(373)	(307)
Intangibles, Net	$ 1,127	$ 1,193

SUMMARY OF SIGNIFICANT ACCOUN_8

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Advance Royalties (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Advance Royalties
Advance royalties, net, affiliates	$ 48,389	$ 41,216
Advance royalties, net, third-parties	12,570	12,685
Total advance royalties, net	$ 60,959	$ 53,901

SUMMARY OF SIGNIFICANT ACCOUN_9

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Taxes
Percentage of qualifying income for tax purposes	90.00%
Assets and Liabilities, Lessee
Operating lease liabilities	$ 14,932
Offsetting right-of-use assets	$ 15,004	$ 17,660
Minimum
Revenue Recognition
Coal shipments, payment period	14 days
Maximum
Revenue Recognition
Coal shipments, payment period	28 days
Defined benefit pension plan
Employee Benefit Plans
Threshold for amortization of unrecognized actuarial gains and losses (as a percent)	10.00%

ACQUISITIONS - Assets and Liabi

ACQUISITIONS - Assets and Liabilities (Details) $ in Thousands	Aug. 02, 2019USD ($)a	Jan. 03, 2019USD ($)a	Dec. 31, 2020USD ($)
Assets acquired and liabilities assumed
Cash and cash equivalents		$ 900
Mineral interests in proved properties		184,032
Mineral interests in unproved properties		291,190
Receivables		9,326
Accounts payable		(1,921)
Net assets acquired		$ 483,527
Wing
ACQUISITION
Royalty acres, net \| a	9,000
Royalty acres, gross \| a	400,000
Cash	$ 144,900
Assets acquired and liabilities assumed
Mineral interests in proved properties	75,071
Mineral interests in unproved properties	67,701
Receivables	2,155
Net assets acquired	144,927
Wing \| Previously Reported
Assets acquired and liabilities assumed
Mineral interests in proved properties	58,084
Mineral interests in unproved properties	84,976
Receivables	1,867
Net assets acquired	144,927
Wing \| Adjustments
Assets acquired and liabilities assumed
Mineral interests in proved properties	16,987
Mineral interests in unproved properties	(17,275)
Receivables	$ 288		$ 300
AllDale I and II
ACQUISITION
Royalty acres, net \| a		43,000
Cash		$ 176,205
Assets acquired and liabilities assumed
Noncontrolling interest fair value		$ 12,300

ACQUISITIONS - Consideration (D

ACQUISITIONS - Consideration (Details) - USD ($) $ in Thousands	Aug. 02, 2019	Jan. 03, 2019	Dec. 31, 2019	Dec. 31, 2020	Jan. 02, 2019
ACQUISITION
Equity method investments			$ 28,529	$ 27,268
Acquisition gain			$ 177,043
AllDale I and II
ACQUISITION
Cash		$ 176,205
Previously held investment		307,322
Total		483,527
Equity method investments					$ 130,300
Acquisition gain		$ 177,000
Wing
ACQUISITION
Cash	$ 144,900

ACQUISITIONS - Pro Forma (Detai

ACQUISITIONS - Pro Forma (Details) - USD ($) $ in Thousands	12 Months Ended
ACQUISITIONS - Pro Forma (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pro forma condensed consolidated income statement
Total revenues, As reported	$ 1,328,129	$ 1,961,720	$ 2,002,857
Net income (loss)	$ (129,051)	406,926	367,470
Wing
Acquisitions, additional information
Revenue of acquired business		4,625
Net income of acquired business		1,291
Pro forma condensed consolidated income statement
Total revenues, Pro forma		1,966,291	2,008,559
Net income, Pro forma		411,217	372,810
AllDale I and II
Acquisitions, additional information
Revenue of acquired business		48,411
Net income of acquired business		$ 18,543
Pro forma condensed consolidated income statement
Total revenues, As reported			2,002,857
Total revenues, Pro forma			2,042,545
Net income (loss)			367,470
Net income, Pro forma			$ 358,741

LONG-LIVED ASSET IMPAIRMENTS (D

LONG-LIVED ASSET IMPAIRMENTS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Aug. 16, 2019	Aug. 15, 2019	Sep. 30, 2018
Asset impairment charges
Asset impairments	$ 24,977	$ 15,190	$ 40,483
Total assets	2,166,016	2,586,694	2,394,748
Dotiki Mine
Asset impairment charges
Asset impairments		15,200	34,300
Total assets			51,000	$ 25,800	$ 35,900	$ 85,300
Accrued payments				$ 5,100
Coal reserves not to exercise lease option
Asset impairment charges
Asset impairments			6,200
Illinois Basin
Asset impairment charges
Asset impairments	25,000
Total assets	$ 1,018,916	$ 1,373,516	$ 1,380,912

GOODWILL IMPAIRMENT (Details)

GOODWILL IMPAIRMENT (Details) - USD ($) $ in Thousands	12 Months Ended
GOODWILL IMPAIRMENT (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
GOODWILL IMPAIRMENT
Goodwill	$ 4,373	$ 136,399
Impairments of goodwill	132,026	0	$ 0
Hamilton mining complex
GOODWILL IMPAIRMENT
Goodwill		$ 132,000
Impairments of goodwill	$ 132,000

INVENTORIES (Details)

INVENTORIES (Details) - USD ($) $ in Thousands	12 Months Ended
INVENTORIES (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
INVENTORIES
Coal	$ 19,756	$ 63,645
Supplies (net of reserve for obsolescence of $5,547 and $5,555, respectively)	36,651	37,660
Total inventories, net	56,407	101,305
Reserve for obsolescence	5,547	5,555
Reduce inventory carrying value to market	$ 3,245	$ 4,895	$ 1,455

PROPERTY, PLANT AND EQUIPMENT -

PROPERTY, PLANT AND EQUIPMENT - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	$ 3,554,090	$ 3,684,008
Less accumulated depreciation, depletion and amortization	(1,753,845)	(1,675,022)
Total property, plant and equipment, net	1,800,245	2,008,986
Mining equipment and processing facilities
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	1,896,324	1,937,642
Land and coal mineral rights
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	454,310	453,237
Coal reserves not subject to depletion	37,500	40,100
Oil and gas mineral interests
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	616,904	618,282
Unproved properties	340,500	376,200
Buildings, office equipment and improvements
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	279,938	304,111
Construction and mine development in progress
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	25,799	86,876
Mine development costs
PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment, at cost	280,815	283,860
Mine development costs	$ 13,100	$ 13,200

LONG-TERM DEBT - Components (De

LONG-TERM DEBT - Components (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Principal
Aggregate maturities of long-term debt	$ 603,780	$ 789,280
Less current maturities	(73,199)	(13,157)
Total long-term debt	530,581	776,123
Unamortized Discount and Debt Issuance Costs
Unamortized debt issuance costs including current and non-current	(11,160)	(7,929)
Total unamortized debt issuance costs	(11,160)	(7,929)
Revolving credit facility
Principal
Aggregate maturities of long-term debt	87,500	255,000
Unamortized Discount and Debt Issuance Costs
Unamortized debt issuance costs including current and non-current	(7,196)	(3,050)
Senior notes due 2025
Principal
Aggregate maturities of long-term debt	400,000	400,000
Unamortized Discount and Debt Issuance Costs
Unamortized debt issuance costs including current and non-current	(3,964)	(4,879)
Securitization Facility
Principal
Aggregate maturities of long-term debt	55,900	73,800
May 2019 equipment financing
Principal
Aggregate maturities of long-term debt	4,956	8,199
November 2019 equipment financing
Principal
Aggregate maturities of long-term debt	42,367	$ 52,281
June 2020 equipment financing
Principal
Aggregate maturities of long-term debt	$ 13,057

LONG-TERM DEBT - Credit Facilit

LONG-TERM DEBT - Credit Facility (Details) $ in Thousands	Mar. 09, 2020USD ($)item	Dec. 31, 2020USD ($)	Jan. 27, 2017USD ($)
Long-Term Debt
Debt issuance costs incurred		$ 6,280
Credit Agreement
Long-Term Debt
Maximum borrowing capacity	$ 537,750
Debt issuance costs incurred		$ 6,300
Number of benchmarks \| item	3
ARLP debt arrangements requirements, fixed charge coverage ratio	1
ARLP debt arrangements requirements, debt to cash flow ratio	2.5
ARLP debt arrangements requirements, cash flow to interest expense ratio	3
ARLP debt arrangements requirements, period over which the ratios are required to be maintained	12 months
Actual debt to cash flow ratio for trailing twelve months		1.53
Actual cash flow to interest expense ratio for trailing twelve months		8.45
Restricted net assets of subsidiaries		$ 240,800
Credit Agreement \| Eurodollar Rate
Long-Term Debt
Effective interest rate (as a percent)		3.01%
Credit Agreement \| Extended Commitment to May 23, 2021
Long-Term Debt
Maximum borrowing capacity	$ 459,500
Credit Agreement \| Credit Agreement, first lien
Long-Term Debt
ARLP debt arrangements requirements, debt to cash flow ratio	1.5
Actual debt to cash flow ratio for trailing twelve months		0.52
Revolving credit facility
Long-Term Debt
Maximum borrowing capacity			$ 494,750
Line of credit facility, available for borrowing		$ 428,500
Annual commitment fee percentage, undrawn portion		0.35%
Letters of credit subfacility
Long-Term Debt
Maximum borrowing capacity	$ 125,000
Letters of credit outstanding		$ 21,800
Swingline subfacility
Long-Term Debt
Maximum borrowing capacity	$ 15,000
Letters of credit - Other
Long-Term Debt
Maximum borrowing capacity		5,000
Letters of credit outstanding		$ 5,000

LONG-TERM DEBT - 2025 Senior No

LONG-TERM DEBT - 2025 Senior Notes (Details) - Senior notes due 2025 $ in Millions	Apr. 24, 2017USD ($)
Issuance of Senior Notes
Principal amount	$ 400
Term	8 years
Interest rate (as a percent)	7.50%

LONG-TERM DEBT - Securitization

LONG-TERM DEBT - Securitization Facility (Details) - Securitization Facility - USD ($) $ in Millions	Jan. 31, 2021	Dec. 31, 2020	Dec. 05, 2014
Long-Term Debt
Maximum borrowing capacity	$ 60		$ 100
Facility outstanding amount		$ 55.9

LONG-TERM DEBT - Equipment fina

LONG-TERM DEBT - Equipment financing and other (Details) $ in Millions	Jun. 05, 2020USD ($)	Nov. 06, 2019USD ($)payment	May 17, 2019USD ($)	Dec. 31, 2020USD ($)
May 2019 Equipment Financing
Long-Term Debt
Principal amount			$ 10
Term			36 months
Effective interest rate (as a percent)			6.25%
November 2019 Equipment Financing
Long-Term Debt
Principal amount		$ 53.1
Term		4 years
Effective interest rate (as a percent)		4.75%
Number of monthly payments \| payment		47
Periodic Payment		$ 1
Balloon payment on maturity		$ 11.6
June 2020 equipment financing
Long-Term Debt
Principal amount	$ 14.7
Term	48 months
Effective interest rate (as a percent)	6.10%
Letters of credit - Other
Long-Term Debt
Credit facility amount				$ 5

LONG-TERM DEBT - Maturities (De

LONG-TERM DEBT - Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Aggregate maturities of long-term debt
2021	$ 73,199
2022	16,071
2023	24,970
2024	89,540
2025	400,000
Aggregate maturities of long-term debt	$ 603,780	$ 789,280

LEASES - Lease expense (Details

LEASES - Lease expense (Details) - USD ($) $ in Thousands	12 Months Ended
LEASES - Lease expense (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
LEASES
Amortization of right-of-use assets	$ 704	$ 14,608
Interest on lease liabilities	377	2,085
Operating lease cost	3,873	9,169
Short-term lease cost	84	464
Variable lease cost	1,375	1,360
Total lease cost	6,413	27,686
Rental expense	$ 5,200	$ 11,000	$ 14,900

LEASES - Cash flow (Details)

LEASES - Cash flow (Details) - USD ($) $ in Thousands	12 Months Ended
LEASES - Cash flow (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
LEASES
Operating cash flows for operating leases	$ 3,870	$ 9,124
Operating cash flows for finance leases	377	891
Financing cash flows for finance leases	8,368	46,725	$ 29,353
Operating leases, Right-of-use assets obtained in exchange for lease obligations	$ 278	$ 25,593

LEASES - Balance Sheet (Details

LEASES - Balance Sheet (Details) - Assets under finance lease - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Lease balance sheet information
Property and equipment finance lease assets, gross	$ 5,485	$ 30,610
Accumulated depreciation	(3,867)	(20,564)
Property and equipment finance lease assets, net	$ 1,618	$ 10,046

LEASES - Weighted Average (Deta

LEASES - Weighted Average (Details)	Dec. 31, 2020	Dec. 31, 2019
LEASES
Weighted average remaining lease term of operating leases	13 years 4 months 24 days	13 years 1 month 6 days
Weighted average remaining lease term of finance leases	3 years 10 months 24 days	1 year 7 months 6 days
Weighted average discount rate of operating leases (as a percent)	6.00%	6.00%
Weighted average discount rate of finance leases (as a percent)	8.00%	6.00%

LEASES - Maturities (Details)

LEASES - Maturities (Details) $ in Thousands	Dec. 31, 2020USD ($)
Operating leases
2021	$ 2,346
2022	2,245
2023	2,061
2024	1,841
2025	1,527
Thereafter	11,838
Total lease payments	21,858
Less imputed interest	(6,926)
Total	14,932
Finance leases
2021	912
2022	912
2023	139
2024	139
2025	139
Thereafter	280
Total lease payments	2,521
Less imputed interest	(297)
Total	$ 2,224

FAIR VALUE MEASUREMENTS (Detail

FAIR VALUE MEASUREMENTS (Details) - Estimated fair value - Significant Observable Inputs (Level 2) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
FAIR VALUE MEASUREMENTS
Long-term debt	$ 518,317	$ 736,206
Total	$ 518,317	$ 736,206

PARTNERS' CAPITAL - Distributio

PARTNERS' CAPITAL - Distributions (Details) - $ / shares	3 Months Ended									12 Months Ended
PARTNERS' CAPITAL - Distributions (Details) - $ / shares	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020
PARTNERS' CAPITAL
Period following quarter end for distribution of available cash										45 days
Quarterly distribution paid (in dollars per unit)	$ 0.400	$ 0.540	$ 0.540	$ 0.535	$ 0.530	$ 0.525	$ 0.520	$ 0.515	$ 0.510

PARTNERS' CAPITAL - Narrative (

PARTNERS' CAPITAL - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Jun. 29, 2018	May 31, 2018	Dec. 31, 2020	Dec. 31, 2018
Partners' capital
Cash contribution by affiliated entity				$ 2,142
Affiliated entity controlled by Mr. Craft
Partners' capital
Contribution of units by affiliated entity (in units)	467,018
Cash contribution by affiliated entity				$ 2,100
MGP \| Intermediate Partnership
Partners' capital
Ownership percentage by general partners			1.0001%
MGP \| Alliance Coal
Partners' capital
Ownership percentage by general partners			0.001%
Limited Partners' Capital
Partners' capital
Issuance of units to Owners of SGP in Simplification Transactions (in units)				1,322,388
Repurchase and retire authorization		$ 100,000
Treasury units retired			$ 93,500
Number of units retired			5,460,639
Repurchase price (in dollars per unit)			$ 17.12
Contribution of units by affiliated entity (in units)				(467,018)
Simplification Transactions \| Intermediate Partnership
Partners' capital
Ownership percentage by general partners		1.0001%
Simplification Transactions \| Alliance Coal
Partners' capital
Ownership percentage by general partners		0.001%
Simplification Transactions \| SGP
Partners' capital
Issuance of units to Owners of SGP in Simplification Transactions (in units)		1,322,388
Simplification Transactions \| MGP \| Intermediate Partnership
Partners' capital
Ownership percentage by general partners		1.0001%
Simplification Transactions \| MGP \| Alliance Coal
Partners' capital
Ownership percentage by general partners		0.001%
Simplification Transactions \| SGP \| Intermediate Partnership
Partners' capital
Ownership percentage by general partners		1.0001%
Simplification Transactions \| SGP \| Alliance Coal
Partners' capital
Ownership percentage by general partners		0.001%

VARIABLE INTEREST ENTITIES - Ca

VARIABLE INTEREST ENTITIES - Cavalier (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 03, 2019
Variable Interest Entities
Contributions			$ 2,142
Distributions paid to Partners	$ 51,753	$ 278,425	$ 275,902
Cavalier Minerals \| Alliance Minerals
Variable Interest Entities
Contributions	143,112
Distributions paid to Partners	89,380
Cavalier Minerals \| Bluegrass Minerals
Variable Interest Entities
Contributions	5,963
Distributions paid to Partners	$ 3,723
Cavalier Minerals \| Bluegrass Minerals
Variable Interest Entities
Incentive distribution of available cash (as a percent)	25.00%
Cavalier Minerals
Variable Interest Entities
Ownership interest in VIE (as a percent)	96.00%
Cavalier Minerals \| Bluegrass Minerals
Variable Interest Entities
Noncontrolling ownership interest (as a percent)	4.00%			4.00%

VARIABLE INTEREST ENTITIES - Al

VARIABLE INTEREST ENTITIES - All Dale III (Detail) - All Dale Minerals III - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 28, 2017
Equity Investments
Other Commitment		$ 30
Distribution after hurdles (as a percent)	25.00%
Specified internal rate of return (as a percent)	10.00%
Percentage of available cash distributed	125.00%
All Dale Minerals III
Equity Investments
Ownership percentage by limited partners	13.90%

VARIABLE INTEREST ENTITIES - WK

VARIABLE INTEREST ENTITIES - WKY CoalPlay (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Coal lease
Variable Interest Entities
Payments	$ 13,819	$ 15,317	$ 10,818

INVESTMENTS - AllDale (Details)

INVESTMENTS - AllDale (Details) - USD ($) $ in Thousands	12 Months Ended
INVESTMENTS - AllDale (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
ASSETS
Beginning balance	$ 28,529
Equity method investment income	907	$ 2,203	$ 22,189
Ending balance	27,268	28,529
All Dale Minerals III
ASSETS
Beginning balance	28,529	28,974	14,182
Contributions			15,600
Equity method investment income	907	2,203	547
Distributions received	(1,895)	(2,648)	(1,355)
Other	(273)
Ending balance	$ 27,268	$ 28,529	$ 28,974

INVESTMENTS - Kodiak (Details)

INVESTMENTS - Kodiak (Details) - USD ($) $ in Thousands	Feb. 08, 2019	Dec. 31, 2019	Jul. 19, 2017
Equity securities without readily determinable fair value
Redemption of preferred interest		$ 134,288
Kodiak
Equity securities without readily determinable fair value
Equity securities	$ 0		$ 100,000
Redemption of preferred interest	135,000
Gain on redemption	$ 11,500

REVENUE FROM CONTRACTS WITH C_3

REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation of revenues (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of revenues
Revenues	$ 1,328,129	$ 1,961,720	$ 2,002,857
Coal sales
Disaggregation of revenues
Revenues	1,232,272	1,762,442	1,844,808
Oil & gas royalties
Disaggregation of revenues
Revenues	42,912	51,735
Transportation revenues
Disaggregation of revenues
Revenues	21,129	99,503	112,385
Other revenues
Disaggregation of revenues
Revenues	31,816	48,040	45,664
Illinois Basin
Disaggregation of revenues
Revenues	770,051	1,236,308	1,321,089
Illinois Basin \| Coal sales
Disaggregation of revenues
Revenues	755,208	1,128,588	1,197,143
Illinois Basin \| Transportation revenues
Disaggregation of revenues
Revenues	12,817	94,686	106,947
Illinois Basin \| Other revenues
Disaggregation of revenues
Revenues	2,026	13,034	16,999
Appalachia
Disaggregation of revenues
Revenues	500,330	644,389	643,965
Appalachia \| Coal sales
Disaggregation of revenues
Revenues	477,064	628,406	635,530
Appalachia \| Transportation revenues
Disaggregation of revenues
Revenues	8,312	4,817	5,435
Appalachia \| Other revenues
Disaggregation of revenues
Revenues	14,954	11,166	3,000
Minerals
Disaggregation of revenues
Revenues	43,141	53,036
Minerals \| Oil & gas royalties
Disaggregation of revenues
Revenues	42,912	51,735
Minerals \| Other revenues
Disaggregation of revenues
Revenues	229	1,301
Other and Corporate
Disaggregation of revenues
Revenues	25,124	56,850	81,492
Other and Corporate \| Coal sales
Disaggregation of revenues
Revenues		22,138	43,393
Other and Corporate \| Transportation revenues
Disaggregation of revenues
Revenues			3
Other and Corporate \| Other revenues
Disaggregation of revenues
Revenues	25,124	34,712	38,096
Elimination
Disaggregation of revenues
Revenues	(10,517)	(28,863)	(43,689)
Elimination \| Coal sales
Disaggregation of revenues
Revenues		(16,690)	(31,258)
Elimination \| Other revenues
Disaggregation of revenues
Revenues	(10,517)	(12,173)	(12,431)
Elimination \| Illinois Basin
Disaggregation of revenues
Revenues		16,690	31,191
Elimination \| Appalachia
Disaggregation of revenues
Revenues			67
Elimination \| Other and Corporate
Disaggregation of revenues
Revenues	$ 10,517	$ 12,173	$ 12,431

REVENUE FROM CONTRACTS WITH C_4

REVENUE FROM CONTRACTS WITH CUSTOMERS - Coal supply contracts (Details) $ in Thousands	Dec. 31, 2020USD ($)
Performance obligations unsatisfied or partially unsatisfied
Total	$ 1,649,637
Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	1,235,182
Appalachia
Performance obligations unsatisfied or partially unsatisfied
Total	414,455
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 972,192
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01 \| Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	$ 653,208
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01 \| Appalachia
Performance obligations unsatisfied or partially unsatisfied
Total	$ 318,984
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 349,125
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01 \| Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	$ 253,654
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01 \| Appalachia
Performance obligations unsatisfied or partially unsatisfied
Total	$ 95,471
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 187,570
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01 \| Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	$ 187,570
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01 \| Appalachia
Performance obligations unsatisfied or partially unsatisfied
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 140,750
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01 \| Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	$ 140,750
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01 \| Appalachia
Performance obligations unsatisfied or partially unsatisfied
Expected timing of satisfaction period	1 year

EARNINGS PER LIMITED PARTNER _3

EARNINGS PER LIMITED PARTNER UNIT - Reconciliation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income (loss) attributable to ARLP	$ (129,220)	$ 399,414	$ 366,604
General partners' equity ownership			(1,560)
LIMITED PARTNERS	(129,220)	399,414	365,044
Distributions to participating securities		(4,254)	(5,114)
Undistributed earnings attributable to participating securities		(2,237)	(1,641)
Net income (loss) attributable to ARLP available to limited partners	$ (129,220)	$ 392,923	$ 358,289
Weighted-average limited partner units outstanding - basic and diluted	127,164,659	128,116,670	130,758,169
Earnings per limited partner unit - basic and diluted	$ (1.02)	$ 3.07	$ 2.74
Anti-dilutive under the treasury stock method (in units)	773,664	1,284,013	1,658,908
MGP \| Intermediate Partnership
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Ownership percentage by general partners	1.0001%
MGP \| Alliance Coal
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Ownership percentage by general partners	0.001%

EMPLOYEE BENEFIT PLANS - Define

EMPLOYEE BENEFIT PLANS - Defined Contribution (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
PSSP
Defined Contribution Plans
Contribution expense	$ 16.1	$ 21.1	$ 19.9

EMPLOYEE BENEFIT PLANS - Benefi

EMPLOYEE BENEFIT PLANS - Benefit Obligations, Plan Assets and Reported Amounts (Details) - Defined benefit pension plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in benefit obligations:
Benefit obligations at beginning of year	$ 136,425	$ 118,958
Interest cost	4,185	4,864	$ 4,462
Actuarial loss	12,396	17,084
Benefits paid	(5,072)	(4,481)
Benefit obligations at end of year	147,934	136,425	118,958
Change in plan assets:
Fair value of plan assets at beginning of year	91,567	75,823
Employer contribution	1,739	5,559
Actual return on plan assets	12,735	14,666
Benefits paid	(5,072)	(4,481)
Fair value of plan assets at end of year	100,969	91,567	$ 75,823
Funded status at the end of year	(46,965)	(44,858)
Amounts recognized in balance sheet:
Non-current liability	(46,965)	(44,858)
Amount recognized in accumulated other comprehensive income consists of:
Prior service cost	(754)	(940)
Net actuarial loss	(46,519)	(45,125)
Amounts recognized in accumulated other comprehensive income	$ (47,273)	$ (46,065)
Weighted-average assumption to determine benefit obligations
Discount rate	2.37%	3.15%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate	3.15%	4.17%
Expected return on plan assets	6.50%	6.50%

EMPLOYEE BENEFIT PLANS - Assump

EMPLOYEE BENEFIT PLANS - Assumptions (Details) - Defined benefit pension plan	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Asset allocation assumptions
Active management premium percentage	1.50%
Asset allocation assumption	100.00%
Actual return on plan assets (as a percent)	14.20%	19.20%
Equity securities
Asset allocation assumptions
Asset allocation assumption	62.00%
Fixed income securities
Asset allocation assumptions
Asset allocation assumption	33.00%
Real estate
Asset allocation assumptions
Asset allocation assumption	5.00%

EMPLOYEE BENEFIT PLANS - Period

EMPLOYEE BENEFIT PLANS - Periodic Benefit Cost and Other Changes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss:
Total defined benefit pension plan adjustments	$ (9,681)	$ (31,122)	$ 5,069
Defined benefit pension plan
Components of net periodic benefit cost:
Interest cost	4,185	4,864	4,462
Expected return on plan assets	(5,861)	(4,932)	(5,784)
Amortization of prior service cost	186	186	186
Amortization of net loss	4,128	3,922	3,608
Net periodic benefit cost	2,638	4,040	2,472
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss:
Net actuarial gain (loss)	(5,522)	(7,350)	(3,326)
Reversal of amortization item: Prior service cost	186	186	186
Reversal of amortization item: net actuarial loss	4,128	3,922	3,608
Total defined benefit pension plan adjustments	(1,208)	(3,242)	468
Net periodic benefit cost	(2,638)	(4,040)	$ (2,472)
Total recognized in net periodic benefit cost and accumulated other comprehensive loss	$ (3,846)	$ (7,282)

EMPLOYEE BENEFIT PLANS - Estima

EMPLOYEE BENEFIT PLANS - Estimated Benefit Payments (Details) - Defined benefit pension plan $ in Thousands	Dec. 31, 2020USD ($)
Estimated future benefit payments
2021	$ 5,629
2022	5,954
2023	6,269
2024	6,488
2025	6,620
2026-2030	34,674
Estimated future benefit payments	65,634
Expected contribution for pension plan in next year	$ 6,500

EMPLOYEE BENEFIT PLANS - Asset

EMPLOYEE BENEFIT PLANS - Asset Allocations (Details) - Defined benefit pension plan	Dec. 31, 2020
Equity securities
Employee Benefit Plans
Target allocation	62.00%
Equity securities \| Minimum
Employee Benefit Plans
Target allocation	45.00%
Equity securities \| Maximum
Employee Benefit Plans
Target allocation	80.00%
Fixed income securities
Employee Benefit Plans
Target allocation	33.00%
Fixed income securities \| Minimum
Employee Benefit Plans
Target allocation	10.00%
Fixed income securities \| Maximum
Employee Benefit Plans
Target allocation	55.00%
Real estate
Employee Benefit Plans
Target allocation	5.00%
Real estate \| Minimum
Employee Benefit Plans
Target allocation	0.00%
Real estate \| Maximum
Employee Benefit Plans
Target allocation	10.00%

EMPLOYEE BENEFIT PLANS - Fair V

EMPLOYEE BENEFIT PLANS - Fair Value of Plan Assets (Details) - Defined benefit pension plan - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	$ 100,969	$ 91,567	$ 75,823
Cash And Cash Equivalents \| Quoted Prices In Active Markets For Identical Assets (Level 1)
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	3,888	2,958
Equities - Global \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	17,549	10,028
Equities - United States \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	31,835	26,812
Equities - United States futures \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	(2,616)
Equities - International developed markets \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	8,920	10,528
Equities - International developed markets futures \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	(4,921)
Equities - International emerging markets \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	6,600	8,410
Equities - International emerging markets futures \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	(975)
Fixed income - Investment grade \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	25,703	26,186
Fixed income - High yield \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	10,056
Fixed income - Emerging markets \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	2,664
Fixed income - Futures \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	(1,265)
Real estate \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	$ 3,531	4,355
Pension Plan assets - Other \| Net asset value per share
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		$ 2,290

COMMON UNIT-BASED COMPENSATIO_3

COMMON UNIT-BASED COMPENSATION PLANS - LTIP Grants Activity (Details) - ARLP LTIP - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended		12 Months Ended
		Dec. 31, 2020	Feb. 29, 2020	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Number of units
Balance at the beginning of the period (in units)				1,603,378		1,828,080		1,694,026
Granted (in units)				1,430,489	[1]	682,155		511,305		424,486
Vested (in units)				(919,524)	[2]	(885,381)	[3]	(331,502)	[3]
Grants canceled (in units)	[4]			(675,302)
Forfeited (in units)				(8,552)		(21,476)		(45,749)
Balance at the end of the period (in units)		1,430,489		1,430,489		1,603,378		1,828,080		1,694,026
Weighted average grant date fair value per unit
Balance at the beginning of the period (in dollars per unit)				$ 20.39		$ 17.18		$ 19.62
Granted (in dollars per unit)				5.02	[1]	18.63		20.40
Vested (in dollars per unit)				$ 21.70	[2]	12.38	[3]	34.61	[3]
Grants canceled (in dollars per unit)	[4]			18.62
Forfeited (in dollars per unit)				$ 20.16		20.84		17.40
Balance at the end of the period (in dollars per unit)		$ 5.02		$ 5.02		$ 20.39		$ 17.18		$ 19.62
Intrinsic value (in dollars)
Intrinsic value of outstanding grants (in dollars)		$ 6,409		$ 6,409		$ 17,349		$ 31,699		$ 33,372
Other information
Common units issued upon vesting			279,622			596,650		191,858
2018 Grants
Number of units
Vested (in units)		(495,038)
2019 Grants
Number of units
Grants canceled (in units)		675,302
2020 Grants
Number of units
Granted (in units)		578,114

[1]	In December 2020, we modified the vesting requirements for certain restricted units that we granted in February 2020 which were determined to be improbable of vesting under the original vesting requirements (the "2020 Grants"). The new vesting requirements make it probable the modified restricted units will vest. Also in December 2020, an additional 578,114 restricted units under these modified vesting requirements were granted. The grant date fair value reflects the modification date fair value for those awards that were modified.
[2]	In February 2020, we issued 279,622 unrestricted common units to LTIP participants as a result of satisfying the vesting requirements for 424,486 restricted units that were granted in 2017. The remaining vested units were settled in cash to satisfy tax withholding obligations of the LTIP participants. In December 2020, we accelerated the vesting requirements for 495,038 restricted units that were granted in 2018 (the "2018 Grants") and settled these restricted units in cash.
[3]	During the years ended December 31, 2019 and 2018, we issued 596,650 and 191,858 , respectively, unrestricted common units to LTIP participants. The remaining vested units were settled in cash to satisfy tax withholding obligations of the LTIP participants.
[4]	In December 2020, 675,302 restricted units that were granted in 2019 (the "2019 Grants") were canceled since it was determined that the vesting requirements for these restricted units were not probable of being satisfied.

COMMON UNIT-BASED COMPENSATIO_4

COMMON UNIT-BASED COMPENSATION PLANS - LTIP Other Information (Details) - USD ($) $ in Thousands, shares in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other information
Cash settlement of grants	$ 2,490
ARLP LTIP
Other information
Unit-based compensation expense	8,100	$ 10,400	$ 10,800
Reduction in compensation expense	1,000
Total unit-based obligation recorded	1,300	$ 20,200
Unrecognized compensation expense (in dollars)	$ 5,800
Weighted-average period for recognition of expense	2 years
Units available for grant	1.7
2018 Grants \| ARLP LTIP
Other information
Incremental compensation cost	$ 5,400
2019 Grants \| ARLP LTIP
Other information
Reversal of cumulative previously recognized expenses	$ 4,800

COMMON UNIT-BASED COMPENSATIO_5

COMMON UNIT-BASED COMPENSATION PLANS - SERP and Directors Compensation Activity (Details) - SERP and Directors' Compensation Plans - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other information
Unit-based compensation expense		$ 700	$ 1,600	$ 1,600
Total unit-based obligation recorded		$ 16,800	$ 16,100
Phantom Share Units (PSUs)
Number of units
Balance at the beginning of the period (in units)		631,365	635,837	561,784
Granted (in units)		129,265	111,012	84,417
Issued (in units)	[1]		(115,484)	(10,364)
Balance at the end of the period (in units)		760,630	631,365	635,837
Weighted average grant date fair value per unit
Balance at the beginning of the period (in dollars per unit)		$ 25.48	$ 27.34	$ 28.64
Granted (in dollars per unit)		5.25	14.50	18.78
Issued (in dollars per unit)	[1]		25.20	27.92
Balance at the end of the period (in dollars per unit)		$ 22.04	$ 25.48	$ 27.34
Intrinsic value (in dollars)
Intrinsic value of outstanding grants (in dollars)		$ 3,408	$ 6,831	$ 11,025	$ 11,067
Other information
Common units issued upon vesting			115,484	7,181

[1]	During the years ended December 31, 2019 and 2018, we issued ARLP common units of 115,484 and 7,181 , respectively, to participants under the SERP and Directors' Deferred Compensation Plan . Units issued in 2018 were net of units settled in cash to satisfy tax withholding obligations.

SUPPLEMENTAL CASH FLOW INFORM_3

SUPPLEMENTAL CASH FLOW INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Paid For:
Interest	$ 44,226	$ 43,093	$ 38,450
Income taxes	12		34
Non-Cash Activity:
Accounts payable for purchase of property, plant and equipment	5,731	14,504	14,585
Right-of-use assets acquired by operating lease	278	25,593
Market value of common units issued under deferred compensation plans before tax withholding requirements	$ 3,837	$ 17,415	$ 6,142

ASSET RETIREMENT OBLIGATIONS -

ASSET RETIREMENT OBLIGATIONS - Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Asset retirement and mine closing liability
Balance at the beginning of the period	$ 137,514	$ 137,114
Accretion expense	4,033	4,087	$ 3,926
Payments	(1,769)	(2,948)
Allocation of liability associated with acquisitions, mine development and change in assumptions	(11,880)	(739)
Balance at the end of the period	$ 127,898	$ 137,514	$ 137,114

ASSET RETIREMENT OBLIGATIONS _2

ASSET RETIREMENT OBLIGATIONS - Estimated Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Estimated payments of asset retirement obligations:
2021	$ 6,411
2022	2,723
2023	2,570
2024	3,317
2025	4,601
Thereafter	210,330
Aggregate undiscounted asset retirement obligations	229,952
Effect of discounting	(102,054)	$ (102,900)
Total asset retirement obligations	127,898	137,514	$ 137,114
Less: current portion	(6,411)
Non-current asset retirement obligations	121,487	133,018
Surety bonds outstanding to performance of reclamation obligations	$ 171,100	$ 181,600

ACCRUED WORKERS' COMPENSATION_3

ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Workers' Compensation Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of changes in the workers' compensation liability
Beginning balance	$ 53,384	$ 49,539
Accruals increase	5,146	7,162
Payments	(8,482)	(11,320)
Interest accretion	1,278	1,606
Valuation loss	3,413	6,397
Ending balance	$ 54,739	$ 53,384
Estimated present value of future obligations and other information
Workers' compensation discount rate	1.95%	2.81%
Letters of credit outstanding to secure workers compensation obligations	$ 95,200	$ 90,200
Other long-term assets
Estimated present value of future obligations and other information
Receivables for traumatic injury claims	$ 7,100	$ 7,700

ACCRUED WORKERS' COMPENSATION_4

ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Benefit Obligations (Details) - Pneumoconiosis benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of the changes in black lung benefit obligations
Benefit obligations at beginning of year	$ 97,683	$ 72,095
Service cost	3,526	2,593	$ 2,525
Interest cost	2,998	3,044	2,542
Actuarial (gain) loss	7,787	23,298
Benefits and expenses paid	(3,498)	(3,347)
Benefit obligations at end of year	$ 108,496	$ 97,683	$ 72,095

ACCRUED WORKERS' COMPENSATION_5

ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Recognized in AOCL (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss:
Total defined benefit pension plan adjustments	$ (9,681)	$ (31,122)	$ 5,069
Pneumoconiosis benefits
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive loss:
Net actuarial gain (loss)	(7,787)	(23,298)	4,599
Reversal of amortization item: Net actuarial (gain) loss	(686)	(4,582)	2
Total defined benefit pension plan adjustments	$ (8,473)	$ (27,880)	$ 4,601
Estimated present value of future obligations and other information
Pneumoconiosis discount rate	2.38%	3.12%	4.13%
Amount recognized in accumulated other comprehensive income consists of:
Net actuarial loss	$ 40,399	$ 31,927	$ 4,047

ACCRUED WORKERS' COMPENSATION_6

ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Recognized in Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS
Workers' compensation claims	$ 54,739	$ 53,384	$ 49,539
Pneumoconiosis benefit claims	108,496	97,683
Total obligations	163,235	151,067
Less current portion	(10,646)	(11,175)
Non-current obligations	$ 152,589	$ 139,892

ACCRUED WORKERS' COMPENSATION_7

ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Expense (Details) - Pneumoconiosis benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accrued Workers Compensation And Pneumoconiosis Benefits
Service cost	$ 3,526	$ 2,593	$ 2,525
Interest cost	2,998	3,044	2,542
Net amortization	(686)	(4,582)	2
Net periodic benefit cost	5,838	1,055	5,069
Workers' compensation expense	12,305	17,541	11,270
Total pneumoconiosis expense	$ 18,143	$ 18,596	$ 16,339

RELATED-PARTY TRANSACTIONS - Af

RELATED-PARTY TRANSACTIONS - Affiliate Royalty Agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction
As of the beginning of period	$ 53,901
As of the end of period	60,959	$ 53,901
Coal lease
Related Party Transaction
As of the beginning of period	41,216	32,645	$ 32,993
Payments	13,819	15,317	10,818
Recoupment	(4,078)	(4,178)	(3,271)
Unrecoupable	(2,568)	(2,568)	(7,895)
As of the end of period	48,389	41,216	32,645
SGP \| Tunnel Ridge
Related Party Transaction
As of the beginning of period	1,500		3,000
Payments	3,000	4,500
Recoupment	(3,000)	(3,000)	(3,000)
As of the end of period	1,500	1,500
WKY CoalPlay \| December 2014 coal lease - Henderson and Union Counties, Kentucky
Related Party Transaction
As of the beginning of period	16,603	14,077	10,684
Payments	3,597	3,597	3,597
Recoupment	(1,022)	(1,071)	(204)
As of the end of period	19,178	16,603	14,077
WKY CoalPlay \| December 2014 coal lease - Webster County, Kentucky
Related Party Transaction
As of the beginning of period			5,356
Payments	2,568	2,568	2,570
Recoupment			(31)
Unrecoupable	(2,568)	(2,568)	(7,895)
WKY CoalPlay \| December 2014 coal lease - Henderson County, Kentucky
Related Party Transaction
As of the beginning of period	12,607	10,086	7,566
Payments	2,522	2,521	2,520
As of the end of period	15,129	12,607	10,086
WKY CoalPlay \| February 2015 coal lease - Henderson and Union Counties, Kentucky
Related Party Transaction
As of the beginning of period	10,506	8,482	6,387
Payments	2,132	2,131	2,131
Recoupment	(56)	(107)	(36)
As of the end of period	$ 12,582	$ 10,506	$ 8,482

RELATED-PARTY TRANSACTIONS - _2

RELATED-PARTY TRANSACTIONS - Affiliate Royalty Agreements - SGP (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Tunnel Ridge
Related Party Transaction
Payments for earned royalties	$ 6.1	$ 7.2	$ 6
Tunnel Ridge \| Joseph W Craft III Foundation \| Tunnel Ridge
Related Party Transaction
Property ownership (as a percent)	0.50%
SGP \| Tunnel Ridge
Related Party Transaction
Annual minimum royalties	$ 3

RELATED-PARTY TRANSACTIONS - WK

RELATED-PARTY TRANSACTIONS - WKY CoalPlay and Cavalier Minerals (Details) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Feb. 28, 2015	Dec. 31, 2014	Dec. 31, 2020	Jan. 03, 2019
WKY CoalPlay \| February 2015 coal lease - Henderson and Union Counties, Kentucky
Related Party Transaction
Initial term of lease	20 years
Percentage of earned royalty on coal sale price	4.00%
Annual minimum royalties	$ 2.1
Period for option to acquire the leased reserves	3 years
Percentage of internal rate of return on purchase price, if leased reserves acquired	7.00%
WKY CoalPlay \| December 2014 coal lease - Henderson and Union Counties, Kentucky
Related Party Transaction
Initial term of lease		20 years
Percentage of earned royalty on coal sale price		4.00%
Annual minimum royalties		$ 3.6
Period for option to acquire the leased reserves		3 years
Percentage of internal rate of return on purchase price, if leased reserves acquired		7.00%
WKY CoalPlay \| December 2014 coal lease - Webster County, Kentucky
Related Party Transaction
Initial term of lease		7 years
Percentage of earned royalty on coal sale price		4.00%
Annual minimum royalties		$ 2.6
Period for option to acquire the leased reserves		3 years
Percentage of internal rate of return on purchase price, if leased reserves acquired		7.00%
Accrued payments			$ 2.6
WKY CoalPlay \| December 2014 coal lease - Henderson County, Kentucky
Related Party Transaction
Annual minimum royalties		$ 2.5
Cavalier Minerals
Related Party Transaction
Ownership interest in VIE (as a percent)			96.00%
Cavalier Minerals \| Bluegrass Minerals
Related Party Transaction
Noncontrolling ownership interest (as a percent)			4.00%	4.00%

COMMITMENTS AND CONTINGENCIES -

COMMITMENTS AND CONTINGENCIES - Purchase Commitments (Details) - USD ($) $ in Millions	Dec. 31, 2020	Mar. 09, 2018
Contractual Commitments
Commitments to purchase coal		$ 2
Commitments for external coal purchases
Contractual Commitments
Commitments to purchase coal	$ 0
Commitments related to planned capital projects
Contractual Commitments
Contractual amount	$ 21

COMMITMENTS AND CONTINGENCIES_2

COMMITMENTS AND CONTINGENCIES - General Litigation (Details) - USD ($) $ in Thousands	Mar. 09, 2018	Dec. 31, 2018
Loss Contingency, Information about Litigation Matters [Abstract]
Settlement per agreement	$ 93,000
Purchase of coal reserves, agreed amount	2,000
Legal and other costs of settlement	13,000
Net gain from settlement	$ 80,000	$ 80,000

COMMITMENTS AND CONTINGENCIES_3

COMMITMENTS AND CONTINGENCIES - Insurance (Details) - Property and casualty insurance $ in Millions	Oct. 01, 2020USD ($)
Commitments And Contingencies
Aggregate maximum insurance limit	$ 100
Insurance deductible	$ 1.5
Waiting period one	75 days
Waiting period	90 days
Overall aggregate deductible	$ 10
Percentage of participating interest in commercial property insurance program	10.00%

CONCENTRATION OF CREDIT RISK _3

CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Major Customers
Revenues	$ 1,328,129	$ 1,961,720	$ 2,002,857
Trade receivables	$ 104,579	$ 161,679
Revenue \| Geographic Concentration Risk
Major Customers
Concentration of risk (as a percent)	3.30%	17.90%	27.80%
Revenue \| Customer Concentration Risk \| Customer A
Major Customers
Revenues	$ 197,379	$ 228,500	$ 219,115
Revenue \| Customer Concentration Risk \| Customer B
Major Customers
Revenues		213,319
Revenue \| Customer Concentration Risk \| Customer C
Major Customers
Revenues	157,271
Revenue \| Customer Concentration Risk \| Customer D
Major Customers
Revenues	137,785
Trade accounts receivable \| Customer Concentration Risk \| Major customers
Major Customers
Trade receivables	$ 32,000	$ 26,300

SEGMENT INFORMATION - General I

SEGMENT INFORMATION - General Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020segment	Feb. 08, 2019USD ($)	Jul. 19, 2017USD ($)
Reportable segment results
Number of reportable segments	3
Number of coal reportable segments	2
Number of mining complex	7
Kodiak
Reportable segment results
Equity securities \| $		$ 0	$ 100

SEGMENT INFORMATION - Segment R

SEGMENT INFORMATION - Segment Results (Details) $ in Thousands	Apr. 26, 2020employee	Apr. 15, 2020employee	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Reportable segment results
Revenues			$ 1,328,129	$ 1,961,720	$ 2,002,857
Segment Adjusted EBITDA Expense			861,249	1,204,896	1,211,800
Segment Adjusted EBITDA			446,489	672,001	715,691
Total assets			2,166,016	2,586,694	2,394,748
Capital expenditures			121,101	305,858	233,480
Gibson County mining complex
Segment Information
Number of employees terminated \| employee		116
Hamilton mining complex
Segment Information
Number of employees terminated \| employee	78
Illinois Basin
Reportable segment results
Revenues			770,051	1,236,308	1,321,089
Segment Adjusted EBITDA Expense			520,324	756,423	796,370
Segment Adjusted EBITDA			236,911	385,200	417,773
Total assets			1,018,916	1,373,516	1,380,912
Capital expenditures			48,648	189,270	166,468
Appalachia
Reportable segment results
Revenues			500,330	644,389	643,965
Segment Adjusted EBITDA Expense			319,730	423,623	398,243
Segment Adjusted EBITDA			172,288	215,950	240,286
Total assets			448,567	500,027	440,518
Capital expenditures			70,960	111,739	64,037
Minerals
Reportable segment results
Revenues			43,141	53,036
Segment Adjusted EBITDA Expense			4,106	7,811
Segment Adjusted EBITDA			39,773	46,997	21,323
Total assets			613,916	643,213	161,312
Other and Corporate
Reportable segment results
Revenues			25,124	56,850	81,492
Segment Adjusted EBITDA Expense			18,543	36,845	52,321
Segment Adjusted EBITDA			6,580	32,911	44,864
Total assets			477,469	541,261	589,010
Capital expenditures			1,493	4,849	2,975
Operating segments
Reportable segment results
Revenues			1,328,129	1,961,720	2,002,857
Operating segments \| Illinois Basin
Reportable segment results
Revenues			770,051	1,219,618	1,289,898
Operating segments \| Appalachia
Reportable segment results
Revenues			500,330	644,389	643,898
Operating segments \| Minerals
Reportable segment results
Revenues			43,141	53,036
Operating segments \| Other and Corporate
Reportable segment results
Revenues			14,607	44,677	69,061
Elimination
Reportable segment results
Revenues			(10,517)	(28,863)	(43,689)
Segment Adjusted EBITDA Expense			(1,454)	(19,806)	(35,134)
Segment Adjusted EBITDA			(9,063)	(9,057)	(8,555)
Total assets			(392,852)	(471,323)	(177,004)
Elimination \| Illinois Basin
Reportable segment results
Revenues				16,690	31,191
Elimination \| Appalachia
Reportable segment results
Revenues					67
Elimination \| Other and Corporate
Reportable segment results
Revenues			$ 10,517	$ 12,173	$ 12,431

SEGMENT INFORMATION - EBITDA Ex

SEGMENT INFORMATION - EBITDA Expense Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of consolidated Segment Adjusted EBITDA Expense to operating expenses (excluding depreciation, depletion and amortization)
Segment Adjusted EBITDA Expense	$ 861,249	$ 1,204,896	$ 1,211,800
Outside coal purchases		(23,357)	(1,466)
Other income (expense)	(1,593)	561	(2,621)
Operating expenses (excluding depreciation, depletion and amortization)	$ 859,656	$ 1,182,100	$ 1,207,713

SEGMENT INFORMATION - EBITDA Re

SEGMENT INFORMATION - EBITDA Reconciliation (Details) - USD ($) $ in Thousands	Mar. 09, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of consolidated Segment Adjusted EBITDA to net income
Consolidated Segment Adjusted EBITDA		$ 446,489	$ 672,001	$ 715,691
General and administrative		(59,806)	(72,997)	(68,298)
Depreciation, depletion and amortization		(313,387)	(309,075)	(280,225)
Settlement gain	$ 80,000			80,000
Asset impairment		(24,977)	(15,190)	(40,483)
Goodwill impairment		(132,026)	0	0
Interest expense, net		(45,478)	(45,496)	(40,059)
Acquisition gain			177,043
Income tax (expense) benefit		(35)	211	(22)
Acquisition gain attributable to noncontrolling interest			(7,083)
NET INCOME (LOSS) ATTRIBUTABLE TO ARLP		(129,220)	399,414	366,604
Noncontrolling interest		169	7,512	866
NET INCOME (LOSS)		$ (129,051)	$ 406,926	$ 367,470

SCHEDULE I - CONDENSED FINANCIA

SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - CONDENSED BALANCE SHEETS (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CURRENT ASSETS:
Cash and cash equivalents	$ 55,574	$ 36,482
Total current assets	245,774	319,585
OTHER ASSETS:
Total other assets	119,997	258,123
TOTAL ASSETS	2,166,016	2,586,694	$ 2,394,748
CURRENT LIABILITIES:
Accrued taxes other than income taxes	25,054	15,768
Total current liabilities	214,605	195,586
Total liabilities	1,093,749	1,321,270
PARTNERS' CAPITAL:
Limited Partners - Common Unitholders 127,195,219 and 126,915,597 units outstanding, respectively	1,148,565	1,331,482
TOTAL LIABILITIES AND PARTNERS' CAPITAL	2,166,016	2,586,694
Parent Company
CURRENT ASSETS:
Cash and cash equivalents	2,174	2,176
Total current assets	2,174	2,176
OTHER ASSETS:
Investments in consolidated subsidiaries	1,146,491	1,329,406
Total other assets	1,146,491	1,329,406
TOTAL ASSETS	1,148,665	1,331,582
CURRENT LIABILITIES:
Accrued taxes other than income taxes	100	100
Total current liabilities	100	100
Total liabilities	100	100
PARTNERS' CAPITAL:
Limited Partners - Common Unitholders 127,195,219 and 126,915,597 units outstanding, respectively	1,148,565	1,331,482
TOTAL LIABILITIES AND PARTNERS' CAPITAL	$ 1,148,665	$ 1,331,582

SCHEDULE I - CONDENSED FINANC_2

SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - CONDENSED BALANCE SHEETS (Parenthetical) (Details) - shares	Dec. 31, 2020	Dec. 31, 2019
Condensed Financial Statements, Captions [Line Items]
Common units outstanding	127,195,219	126,915,597
Parent Company
Condensed Financial Statements, Captions [Line Items]
Common units outstanding	127,195,219	126,915,597

SCHEDULE I - CONDENSED FINANC_3

SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - CONDENSED STATEMENT OF OPERATIONS (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
EXPENSES:
General and administrative	$ 59,806	$ 72,997	$ 68,298
Total operating expenses	1,410,981	1,702,222	1,630,570
INCOME (LOSS) FROM OPERATIONS	(82,852)	259,498	372,287
Interest income	135	379	159
NET INCOME (LOSS) ATTRIBUTABLE TO ARLP	(129,220)	399,414	366,604
NET INCOME (LOSS) ATTRIBUTABLE TO ARLP
GENERAL PARTNER			1,560
LIMITED PARTNERS	$ (129,220)	$ 399,414	$ 365,044
EARNINGS PER LIMITED PARTNER UNIT - BASIC AND DILUTED	$ (1.02)	$ 3.07	$ 2.74
WEIGHTED-AVERAGE NUMBER OF UNITS OUTSTANDING - BASIC AND DILUTED	127,164,659	128,116,670	130,758,169
Parent Company
EXPENSES:
General and administrative		$ 41	$ 30
Total operating expenses		41	30
INCOME (LOSS) FROM OPERATIONS		(41)	(30)
Interest income	$ 24	34	22
Equity in earnings of consolidated subsidiaries	(129,244)	399,421	366,612
NET INCOME (LOSS) ATTRIBUTABLE TO ARLP	(129,220)	399,414	366,604
NET INCOME (LOSS) ATTRIBUTABLE TO ARLP
GENERAL PARTNER			1,560
LIMITED PARTNERS	$ (129,220)	$ 399,414	$ 365,044
EARNINGS PER LIMITED PARTNER UNIT - BASIC AND DILUTED	$ (1.02)	$ 3.07	$ 2.74
WEIGHTED-AVERAGE NUMBER OF UNITS OUTSTANDING - BASIC AND DILUTED	127,164,659	128,116,670	130,758,169

SCHEDULE I - CONDENSED FINANC_4

SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - CONDENSED STATEMENT OF CASH FLOWS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
CASH FLOWS FROM OPERATING ACTIVITIES	$ 400,645	$ 514,895	$ 694,345
CASH FLOWS FROM FINANCING ACTIVITIES:
Distributions paid to Partners	(51,753)	(278,425)	(275,902)
Net cash used in financing activities	(256,429)	(234,447)	(211,702)
NET CHANGE IN CASH AND CASH EQUIVALENTS	19,092	(207,668)	237,394
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	36,482	244,150	6,756
CASH AND CASH EQUIVALENTS AT END OF PERIOD	55,574	36,482	244,150
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES:
CASH FLOWS FROM OPERATING ACTIVITIES	51,751	278,308	275,924
CASH FLOWS FROM FINANCING ACTIVITIES:
Distributions paid to Partners	(51,753)	(278,425)	(275,902)
Net cash used in financing activities	(51,753)	(278,425)	(275,902)
NET CHANGE IN CASH AND CASH EQUIVALENTS	(2)	(117)	22
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	2,176	2,293	2,271
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ 2,174	$ 2,176	$ 2,293

SCHEDULE I - CONDENSED FINANC_5

SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Cash contribution by affiliated entity			$ 2,142
Parent Company
Condensed Financial Statements, Captions [Line Items]
Cash contribution by affiliated entity	$ 51,800	$ 278,400	$ 275,900