COMPENSATION PLANS - LTIP Grants Activity (Details) - ARLP LTIP - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of non-vested grants (in units)
Balance at the beginning of the period (in units)	1,604,748	939,793	843,340
Granted (in units)	475,310	960,992	303,165
Vested (in units)	(350,516)	(284,272)	(202,778)
Forfeited (in units)	(35,516)	(11,765)	(3,934)
Balance at the end of the period (in units)	1,694,026	1,604,748	939,793
Weighted average grant date fair value per unit
Balance at the beginning of the period (in dollars per unit)	$ 23.19	$ 36.80	$ 37.16
Granted (in dollars per unit)	23.17	12.38	37.18
Vested (in dollars per unit)	40.73	31.51	38.85
Forfeited (in dollars per unit)	20.01	26.39	36.49
Balance at the end of the period (in dollars per unit)	$ 19.62	$ 23.19	$ 36.80
Intrinsic value (in dollars)
Intrinsic value of outstanding grants (in dollars)	$ 33,372	$ 36,027	$ 12,678	$ 36,306
Other information
Common units issued upon vesting	222,011	176,319	128,150