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COMPENSATION PLANS - LTIP Other Information (Details) - ARLP LTIP - USD ($)
$ in Millions
12 Months Ended
Feb. 08, 2017
Jan. 25, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Other information          
Common units issued upon vesting     176,319 128,150 128,610
Share based compensation expense     $ 12.7 $ 11.2 $ 9.6
Total unit based obligation recorded     25.1 $ 21.4  
Unrecognized compensation expense     $ 11.7    
Weighted-average period for recognition of expense     1 year 9 months 18 days    
Units for which vesting requirements were deemed satisfied     284,272 202,778 202,742
Forfeited (in units)     11,765 3,934 5,604
Granted (in units)     960,992 303,165 356,154
Units available for grant 2,500,000        
2014 Grants          
Other information          
Common units issued upon vesting 222,011        
Units for which vesting requirements were deemed satisfied   350,516      
Forfeited (in units)   5,638      
2017 Grants          
Other information          
Additional grants authorized (in units)   472,890      
Granted (in units)   462,890