EARNINGS PER LIMITED PARTNER UNIT - Reconciliation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income (loss) attributable to ARLP	$ 27,199	$ 39,084	$ (164,248)	$ 373,582
Distributions to participating securities		$ (1,133)		(3,360)
Undistributed earnings attributable to participating securities	$ (581)			$ (2,655)
Weighted-average limited partner units outstanding - basic and diluted	127,195,219	128,391,191	127,154,398	128,311,609
Earnings per limited partner unit - basic and diluted	$ 0.21	$ 0.30	$ (1.29)	$ 2.86
Anti-dilutive under the treasury stock method (in units)	661,000	1,133,000	656,000	1,216,000
ARLP
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income (loss) attributable to ARLP	$ 27,199	$ 39,084	$ (164,248)	$ 373,582
Net income (loss) attributable to ARLP available to limited partners	$ 26,618	$ 37,951	$ (164,248)	$ 367,567

Aggregate amount of net income available to limited partners after allocation to participating securities.

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Amount of undistributed earnings (loss) allocated to participating securities for the basic and diluted earnings (loss) per share or per unit calculations under the two-class method.

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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Amount of paid and unpaid cash, stock, and paid-in-kind dividends declared for award under share-based payment arrangement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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The amount of net income or loss for the period per each limited partnership and general partnership unit in instances when basic and diluted earnings per unit are the same amount and aggregated on the face of the financial statements. Basic earnings per unit is the amount of net income or loss for the period per unit outstanding during the reporting period. Diluted earnings per unit includes the amount of net income or loss for the period available to each limited and general partnership unit outstanding during the reporting period and to each unit that would have been outstanding assuming the issuance of units for all dilutive potential units outstanding during the reporting period.

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The average number of limited partnership and general partnership units issued and outstanding that are used in calculating basic and diluted earnings per limited partnership and general partnership unit.

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