COMPENSATION PLANS - LTIP Grants Activity (Details) - ARLP LTIP - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Summary of non-vested grants (in units)
Balance at the beginning of the period (in units)	1,604,748
Granted (in units)	475,310
Vested (in units)	(350,516)
Forfeited (in units)	(35,516)
Balance at the end of the period (in units)	1,694,026
Weighted average grant date fair value per unit
Balance at the beginning of the period (in dollars per unit)	$ 23.19
Granted (in dollars per unit)	23.17
Vested (in dollars per unit)	40.73
Forfeited (in dollars per unit)	20.01
Balance at the end of the period (in dollars per unit)	$ 19.62
Intrinisic value (in dollars)
Intrinsic value of outstanding grants (in dollars)	$ 32,779	$ 36,027
Other information
Common units issued upon vesting	222,011