INCOME TAX - Reconciliation of Effective Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Computed tax benefit at statutory rate	$ (82,022)	$ (58,066)	$ (4,390)
Increase (decrease) in tax expense resulting from:
Tax depletion in excess of basis	(5,317)	(9,273)	(6,187)
Non-deductible acquisition costs	0	2,979	0
Non-deductible interest expense	(6,488)	(4,174)	1,167
State income taxes, net	(6,184)	(9,706)	3,696
Adjustments to deferred tax assets and liabilities attributable to prior years	4,244	0	489
Change in valuation allowance for net deferred tax assets	132,338	77,771	15
Release of valuation allowance arising from amalgamation	(32,635)	0	0
Indian Coal Tax Credits	(13,756)	(15,205)	92
Change in Canadian rate	(3,081)	0	0
Kemmerer deferred tax asset removal	(13,238)	0	0
Uncertain tax positions	3,994	0	0
Other, net	6,622	15,906	825
Income tax benefit	$ (19,767)	$ 232	$ (4,782)