Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax liabilities [Abstract]
Hedge contracts	$ 0	$ 0	$ 9,578
Assets held for sale	0	3,390	0
Deferred Tax Liabilities, Other	2,834	4,431	4,042
Total deferred tax liabilities	2,834	7,821	13,620
Deferred tax assets [Abstract]
U.S. full cost pool	20,011	48,436	35,689
Deferred Tax Assets, Capital Loss Carryforwards	3,015	7,361	7,767
Depletion carryforward	3,174	5,216	5,558
U.S. net operating loss carryforward	53,545	80,670	67,531
Alternative minimum tax credit	757	757	757
Hedge contracts	2,777	3,135	0
Total deferred tax assets	83,279	145,575	117,302
Valuation allowance for deferred tax assets	(80,445)	(137,754)	(103,682)
Net deferred tax assets	2,834	7,821	13,620
Net deferred tax	0	0	0
Current [Abstract]
Federal	0	0	(242)
State	0	0	(37)
Current provision (benefit) for income taxes	0	0	(279)
Deferred [Abstract]
Federal	0	0	0
Deferred provision (benefit) for income taxes	0	0	0
Reconciliation of income tax computed at U.S federal statutory tax rates [Abstract]
Tax (expense) benefit at U.S. statutory rates	(5,602)	33,732	44,586
(Increase) decrease in deferred tax asset valuation allowance	57,309	(34,072)	(43,596)
Accrual of prior year federal taxes (2009 and 2013)	0	0	37
Permanent differences	(1,134)	(1,133)	(1,371)
Return to provision estimate revision	2,494	1,473	0
Change in deferred tax rate	(53,125)	0	0
Other	58	0	55
Income tax (expense) benefit	$ 0	$ 0	$ 279
U.S statutory rate (in hundredths)	35.00%
U.S. Tax [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 255,000
Expiration dates of operating loss carryforwards	Dec. 31, 2037