EMPLOYEE BENEFIT PLANS - Periodic Benefit Cost and Other Changes (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive income:
Total recognized adjustments		$ (3,983)	$ 1,290	$ (26,128)
Pension Plan
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive income:
Prior service credit		(1,498)
Net actuarial loss		(2,589)	(863)	(23,821)
Reversal of amortization item: net actuarial (gain) loss	[1]	2,952	3,354	773
Total recognized adjustments		(1,135)	2,491	(23,048)
ARLP | Pension Plan
Components of net periodic benefit cost:
Service cost		2,205	2,473	2,174
Interest cost		4,493	4,296	4,074
Expected return on plan assets		(5,138)	(5,590)	(5,475)
Amortization of net loss		2,952	3,354	773
Net periodic benefit cost		4,512	4,533	1,546
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive income:
Prior service credit		(1,498)
Net actuarial loss		(2,589)	(863)
Reversal of amortization item: net actuarial (gain) loss		2,952	3,354
Total recognized adjustments		(1,135)	2,491
Net periodic benefit cost		(4,512)	(4,533)	$ (1,546)
Total recognized in net periodic benefit cost and accumulated other comprehensive (loss) income		$ (5,647)	$ (2,042)

[1] Amortization of actuarial gain or loss is included in the computation of net periodic benefit cost (see Notes 13 and 17 for additional details).