EMPLOYEE BENEFIT PLANS - Periodic Benefit Cost and Other Changes (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive income:
Total recognized adjustments		$ (13,400)	$ (3,983)	$ 1,290
Pension Plan
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive income:
Prior service cost			(1,498)
Net actuarial loss		(6,610)	(2,589)	(863)
Reversal of amortization item: Prior service cost	[1]	186
Reversal of amortization item: net actuarial (gain) loss	[1]	3,054	2,952	3,354
Total recognized adjustments		(3,370)	(1,135)	2,491
ARLP | Pension Plan
Components of net periodic benefit cost:
Service cost			2,205	2,473
Interest cost		4,587	4,493	4,296
Expected return on plan assets		(4,978)	(5,138)	(5,590)
Amortization of prior service cost		186
Amortization of net loss		3,054	2,952	3,354
Net periodic benefit cost		2,849	4,512	4,533
Other changes in plan assets and benefit obligations recognized in accumulated other comprehensive income:
Prior service cost			(1,498)
Net actuarial loss		(6,610)	(2,589)
Reversal of amortization item: Prior service cost		186
Reversal of amortization item: net actuarial (gain) loss		3,054	2,952
Total recognized adjustments		(3,370)	(1,135)
Net periodic benefit cost		(2,849)	(4,512)	$ (4,533)
Total recognized in net periodic benefit cost and accumulated other comprehensive (loss) income		$ (6,219)	$ (5,647)

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