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COMPENSATION PLANS (Details) - ARLP LTIP - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended 36 Months Ended
Feb. 11, 2015
Jan. 02, 2015
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Jan. 26, 2015
Compensation Plans                
Common units issued upon vesting 128,150              
Share based compensation expense     $ 2.9 $ 2.5 $ 5.5 $ 4.6    
Units available for grant     3,700,000   3,700,000      
Unrecognized compensation expense     $ 17.9   $ 17.9      
Weighted-average period for recognition of expense         1 year 6 months      
Total unit based obligation recorded     15.6   $ 15.6      
Phantom Share Units (PSUs)                
Compensation Plans                
Additional grants authorized (in units)               314,019
Units granted         303,165      
Fair value as intrinsic value at date of grant (in dollars per unit)         $ 37.18      
Intrinsic value of outstanding grants     $ 23.5   $ 23.5      
Phantom Share Units (PSUs) | 2012 Grants                
Compensation Plans                
Units for which vesting requirements were deemed satisfied   202,778            
Forfeitures (in units)             11,450