Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred income tax assets:
Net operating loss and credit carryforwards	$ 395,526	$ 896,181
Inventory	1,083	851
Asset retirement obligations	23,765	37,425
Black lung obligations	7,991	10,826
Accrued expenses	561	2,501
Other	197	1,963
Total	429,123	949,747
Less: valuation allowance for deferred income tax assets	(312,493)	(767,290)	$ 0
Net deferred income tax assets	116,630	182,457
Deferred income tax liabilities:
Prepaid expenses	(9,416)	(8,073)
Property, plant and equipment	(105,715)	(174,098)
Other	(1,757)	(2,230)
Total deferred income tax liabilities	(116,888)	(184,401)
Net deferred income tax liability	$ (258)	$ (1,944)