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COMPENSATION PLANS - LTIP Other Information (Details) - ARLP LTIP - USD ($) $ in Millions				12 Months Ended
	Jan. 22, 2022	Feb. 10, 2020	Jan. 23, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other information
Unit-based compensation expense				$ 10.4	$ 10.8	$ 11.0
Total unit-based obligation recorded				20.2	$ 20.8
Unrecognized compensation expense (in dollars)				$ 12.5
Weighted-average period for recognition of expense				1 year 2 months 12 days
Units for which vesting requirements were deemed satisfied				885,381	331,502	350,516
Forfeitures (in units)				21,476	45,749	35,516
Common units issued upon vesting				596,650	191,858	222,011
Units available for grant				1,000,000.0
Units granted				682,155	511,305	475,310
2017 Grants
Other information
Units for which vesting requirements were deemed satisfied			424,486
Forfeitures (in units)			50,824
Common units issued upon vesting		279,622
Additional grants authorized (in units)	965,035
Units granted	950,035

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Amount of partnership capital allocated for share-based obligations.

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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