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COMMON UNIT-BASED COMPENSATION PLANS - LTIP Other (Details) - ARLP LTIP - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2022
Jun. 30, 2021
Jun. 30, 2022
Jun. 30, 2021
Other information        
Unit-based compensation expense $ 2.1 $ 1.4 $ 4.4 $ 2.1
Total unit-based obligation recorded 11.0   11.0  
Unrecognized compensation expense (in dollars) $ 14.5   $ 14.5  
Weighted-average period for recognition of expense     1 year 3 months 18 days