Income Taxes (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ 29,537	$ 1,494	$ 1,571
State	(67)	(252)	535
Total current	29,470	1,242	2,106
Deferred:
Federal	0	915	(76,300)
State	0	56	(3,186)
Total deferred	0	971	(79,486)
Total income tax benefit (expense)	(29,470)	(2,213)	77,380
Deferred income tax assets:
Accrued expense and liabilities	17,530	26,746
Pension and other postretirement benefits	5,898	8,621
Accrued reclamation and mine closure costs	22,229	34,326
Contract rights	38,793	69,352
Debt Restructuring	12,762	25,183
Mark-to-market gain (loss)	52	(191)
AMT Credit carryforward	0	31,625
Net operating loss carry forward	55,160	54,764
Other	866	7,606
Total deferred income tax assets	153,290	258,032
Less valuation allowance	(52,010)	(99,638)	$ (119,000)
Net deferred income tax asset	101,280	158,394
Deferred income tax liabilities:
Property, Plant and Equipment	(33,420)	(37,954)
Inventories	(11,521)	(17,435)
Mineral rights	(44,878)	(78,819)
Throughput payments	(3,604)	(12,928)
Other	(7,857)	(11,258)
Total deferred income tax liabilities	(101,280)	(158,394)
Net deferred income tax assets (liabilities)	0	0
Deferred tax valuation allowance decrease	(47,600)	(19,300)
State And Local Jurisdiction [Member]
Tax Credit Carryforward [Line Items]
Operating loss carryforwards that expire	68,900	45,200
Domestic Country [Member]
Tax Credit Carryforward [Line Items]
Operating loss carryforwards that expire	$ 247,400	151,600
Deferred Tax Assets Tax Credit Carryforwards		$ 31,600