COMPENSATION PLANS - LTIP Grants Activity (Details) - ARLP LTIP - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of units
Balance at the beginning of the period (in units)	1,828,080	1,694,026	1,604,748
Granted (in units)	682,155	511,305	475,310
Vested (in units)	(885,381)	(331,502)	(350,516)
Forfeited (in units)	(21,476)	(45,749)	(35,516)
Balance at the end of the period (in units)	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)	$ 17.18	$ 19.62	$ 23.19
Granted (in dollars per unit)	18.63	20.40	23.17
Vested (in dollars per unit)	12.38	34.61	40.73
Forfeited (in dollars per unit)	20.84	17.40	20.01
Balance at the end of the period (in dollars per unit)	$ 20.39	$ 17.18	$ 19.62
Intrinsic value (in dollars)
Intrinsic value of outstanding grants (in dollars)	$ 17,349	$ 31,699	$ 33,372	$ 36,027
Other information
Common units issued upon vesting	596,650	191,858	222,011

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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Number of shares issued under share-based payment arrangement.

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Intrinsic value of outstanding award under share-based payment arrangement. Excludes share and unit options.

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