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COMPENSATION PLANS - ARLP Long-Term Incentive Plan (Details) - ARLP LTIP - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended 36 Months Ended
Feb. 11, 2016
Jan. 01, 2016
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Jan. 22, 2016
Compensation Plans                
Common units issued upon vesting 176,319              
Common unit-based based compensation expense     $ 3.1 $ 2.8 $ 9.0 $ 8.3    
Units available for grant     2,800,000   2,800,000      
Unrecognized compensation expense     $ 14.6   $ 14.6      
Weighted-average period for recognition of expense         1 year 7 months 6 days      
Total unit based obligation recorded     21.5   $ 21.5      
Phantom Share Units (PSUs)                
Compensation Plans                
Additional grants authorized (in units)               969,028
Units granted         960,992      
Fair value as intrinsic value at date of grant (in dollars per unit)         $ 12.38      
Intrinsic value of outstanding grants     $ 35.7   $ 35.7      
Phantom Share Units (PSUs) | 2013 Grants                
Compensation Plans                
Units for which vesting requirements were deemed satisfied   284,272            
Forfeitures (in units)             9,178