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COMPENSATION PLANS - ARLP Long-Term Incentive Plan (Details) - ARLP LTIP - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended 24 Months Ended 36 Months Ended
Feb. 11, 2016
Jan. 22, 2016
Jan. 01, 2016
Feb. 11, 2015
Jan. 01, 2015
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Compensation Plans                      
Common units issued upon vesting 176,319     128,150              
Units available for grant 3,800,000                    
Common unit-based based compensation expense           $ 11.2 $ 9.6 $ 7.4      
Total unit based obligation recorded           $ 21.4 $ 17.9   $ 21.4 $ 21.4 $ 17.9
Phantom Share Units (PSUs)                      
Compensation Plans                      
Units for which vesting requirements were deemed satisfied           202,778          
Forfeitures (in units)           3,934          
Additional grants authorized (in units)   969,028                  
Units granted   959,028       303,165 356,154        
Fair value as intrinsic value at date of grant (in dollars per unit)           $ 37.18 $ 40.72 $ 31.51      
Phantom Share Units (PSUs) | 2013 Grants                      
Compensation Plans                      
Units for which vesting requirements were deemed satisfied     284,272                
Forfeitures (in units)                   9,178  
Phantom Share Units (PSUs) | 2012 Grants                      
Compensation Plans                      
Units for which vesting requirements were deemed satisfied         202,778            
Forfeitures (in units)                     11,450
Phantom Share Units (PSUs) | 2015 and 2014 Grants                      
Compensation Plans                      
Forfeitures (in units)                 12,976