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COMPENSATION PLANS - LTIP Other Information (Details) - ARLP LTIP - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Jan. 01, 2020
Other information          
Unit-based compensation expense $ 1.6 $ 2.7 $ 0.9 $ 5.1  
Reversal of cumulative previously recognized expenses     4.8    
Total unit-based obligation recorded 9.7   $ 9.7    
Units expected to vest (in units)     1,439,790    
Units expected to vest (in dollars per units)     $ 13.33    
Intrinsic value of unvested units 4.7   $ 4.7    
Unrecognized compensation expense (in dollars) $ 9.5   $ 9.5    
Weighted-average period for recognition of expense     1 year 9 months 18 days    
Units available for grant 1,700,000   1,700,000   1,000,000.0
Units for which vesting requirements were deemed satisfied     424,486    
Forfeitures (in units)     13,835    
Common units issued upon vesting     279,622    
Units granted     950,035