EMPLOYEE BENEFIT PLANS - Fair Value of Plan Assets (Details) - Pension Plan - ARLP - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Benefit Plans
Defined benefit plan, fair value of plan assets (a)	$ 81,981	$ 71,412	$ 68,445
Cash And Cash Equivalents | Level 1
Employee Benefit Plans
Defined benefit plan, fair value of plan assets (a)	1,439	1,137
Equities - U.S. large-cap
Employee Benefit Plans
Commingled investment funds measured at net asset value (b)	26,031	21,082
Equities - U.S. small-cap
Employee Benefit Plans
Commingled investment funds measured at net asset value (b)	6,120	6,531
Equities - International developed markets
Employee Benefit Plans
Commingled investment funds measured at net asset value (b)	15,015	11,074
Equities - International emerging markets
Employee Benefit Plans
Commingled investment funds measured at net asset value (b)	6,528	4,614
Fixed income - Investment grade
Employee Benefit Plans
Commingled investment funds measured at net asset value (b)	13,546	16,823
Fixed income - High yield
Employee Benefit Plans
Commingled investment funds measured at net asset value (b)	4,325	4,543
Real estate
Employee Benefit Plans
Commingled investment funds measured at net asset value (b)	3,754	4,259
Other
Employee Benefit Plans
Commingled investment funds measured at net asset value (b)	$ 5,223	$ 1,349