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COMPENSATION PLANS - LTIP Grants Activity (Details) - ARLP LTIP - USD ($)
$ in Millions
12 Months Ended
Jan. 22, 2016
Dec. 31, 2015
Dec. 31, 2014
Summary of non-vested LTIP grants      
Unrecognized compensation expense     $ 12.1
Weighted-average period for recognition of expense     1 year 6 months
Phantom Share Units (PSUs)      
Summary of non-vested LTIP grants      
Balance at the beginning of the period (in units)   843,340  
Granted (in units) 959,028 303,165 356,154
Vested (in units)   (202,778)  
Forfeited (in units)   (3,934)  
Balance at the end of the period (in units)   939,793 843,340
Intrinsic value of outstanding grants     $ 12.7