Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of the provision for income taxes
Federal	$ 5,698,000	$ 2,932,000	$ 0
State	3,398,000	1,539,000	1,571,000
Deferred portion of income tax benefit	(192,066,000)	(448,643,000)	(276,722,000)
Total income tax benefit	$ (182,970,000)	$ (444,172,000)	$ (275,151,000)
Effective tax rate (as a percent)	53.20%	37.00%	38.10%
Deferred income taxes [Abstract]
Deferred tax liabilities, oil and gas properties	$ 142,467,000	$ 518,394,000
Deferred tax liabilities, other	3,412,000	7,733,000
Total deferred tax liabilities	145,879,000	526,127,000
Deferred tax assets, derivative liabilities	29,463,000	31,349,000
Deferred tax assets, credit carryover	22,537,000	12,448,000
Deferred tax assets, pension	7,986,000	10,366,000
Deferred tax assets, federal and state tax net operating loss carryovers	3,867,000	151,343,000
Deferred tax assets, stock-compensation	3,545,000	10,083,000
Deferred tax assets, other liabilities	1,470,000	201,000
Total deferred tax assets	68,868,000	215,790,000
Deferred tax assets, valuation allowance	(2,978,000)	(5,335,000)
Net deferred tax assets	65,890,000	210,455,000
Total net deferred tax liabilities	79,989,000	315,672,000
Proceeds from income tax refunds	5,500,000
Net R&D tax credit recorded for IRS settlement		2,000,000
Reconciliation of unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, Beginning balance	446,000	2,782,000	$ 1,582,000
Unrecognized tax benefits, additions for tax positions of prior years	0	9,000	1,200,000
Unrecognized tax benefits, settlements	0	(2,345,000)	0
Unrecognized tax benefits, Ending balance	$ 446,000	446,000	$ 2,782,000
Tax year 2017 [Member]
Deferred income taxes [Abstract]
Federal enacted statutory income tax rate, percent	35.00%
Future tax years [Member]
Deferred income taxes [Abstract]
Federal enacted statutory income tax rate, percent	21.00%
State and local jurisdiction [Member]
Deferred income taxes [Abstract]
Current state income tax payable	$ 3,009,000	1,181,000
Net operating loss carryforwards	71,800,000
State tax credits [Member] | State and local jurisdiction [Member]
Deferred income taxes [Abstract]
Tax credit carryforwards	283,000
Internal Revenue Service (IRS) [Member] | Domestic tax authority [Member]
Deferred income taxes [Abstract]
Current federal income tax refundable	37,000	$ 644,000
Net operating loss carryforwards	1,300,000
Internal Revenue Service (IRS) [Member] | R&D tax credit carryforwards [Member] | Domestic tax authority [Member]
Deferred income taxes [Abstract]
Tax credit carryforwards	7,200,000
Internal Revenue Service (IRS) [Member] | Alternative minimum tax credit carryforward [Member] | Domestic tax authority [Member]
Deferred income taxes [Abstract]
Tax credit carryforwards	$ 15,700,000