EMPLOYEE BENEFIT PLANS - Benefit Obligations, Plan Assets and Reported Amounts (Details) - ARLP - Pension Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in benefit obligations:
Benefit obligations at beginning of year	$ 107,476	$ 109,626
Service cost	2,205	2,473	$ 2,174
Interest cost	4,493	4,296	4,074
Actuarial (gain) loss	901	(6,420)
Benefits paid	(3,091)	(2,499)
Plan amendments	1,498
Benefit obligations at end of year	113,482	107,476	109,626
Change in plan assets:
Fair value of plan assets at beginning of year	68,445	69,521
Employer contribution	2,608	3,116
Actual return on plan assets	3,450	(1,693)
Benefits paid	(3,091)	(2,499)
Fair value of plan assets at end of year	71,412	68,445	$ 69,521
Funded status at the end of year	(42,070)	(39,031)
Amounts recognized in balance sheet:
Non-current liability	(42,070)	(39,031)
Total amounts recognized in balance sheet	(42,070)	(39,031)
Amounts recognized in accumulated other comprehensive income consist of:
Prior service credit	(1,498)
Net actuarial loss	(38,424)	(38,787)
Amounts recognized in accumulated other comprehensive income	$ (39,922)	$ (38,787)
Weighted-average assumptions to determine benefit obligations
Discount rate	4.06%	4.27%
Expected rate of return on plan assets	7.00%	7.50%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate	4.27%	3.92%
Expected return on plan assets	7.50%	8.00%