Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of the provision for income taxes
Federal	$ 0	$ 5,698,000	$ 2,932,000
State	1,662,000	3,398,000	1,539,000
Deferred portion of income tax expense (benefit)	141,708,000	(192,066,000)	(448,643,000)
Total income tax expense (benefit)	$ 143,370,000	$ (182,970,000)	$ (444,172,000)
Effective tax rate (as a percent)	22.00%	53.20%	37.00%
Deferred income taxes [Abstract]
Deferred tax liabilities, oil and gas properties	$ 218,094,000	$ 142,467,000
Deferred tax liabilities, derivatives assets	35,247,000	0
Deferred tax liabilities, other	4,812,000	3,412,000
Total deferred tax liabilities	258,153,000	145,879,000
Deferred tax assets, derivative liabilities	0	29,463,000
Deferred tax assets, credit carryover	22,554,000	22,537,000
Deferred tax assets, pension	6,427,000	7,986,000
Deferred tax assets, federal and state tax net operating loss carryovers	4,217,000	3,867,000
Deferred tax assets, stock compensation	3,263,000	3,545,000
Deferred tax assets, other liabilities	1,497,000	1,470,000
Total deferred tax assets	37,958,000	68,868,000
Deferred tax assets, valuation allowance	(3,083,000)	(2,978,000)
Net deferred tax assets	34,875,000	65,890,000
Total net deferred tax liabilities	223,278,000	79,989,000
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount	1,000,000
Proceeds from income tax refunds	5,900,000	5,500,000
Reconciliation of unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, Beginning balance	446,000	446,000	$ 2,782,000
Unrecognized tax benefits, additions for tax positions of prior years	0	0	9,000
Unrecognized tax benefits, settlements	0	0	(2,345,000)
Unrecognized tax benefits, Ending balance	$ 446,000	446,000	$ 446,000
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	$ 1,331,000	3,009,000
Net operating loss carryforwards	79,700,000
State tax credits [Member] | State and local jurisdiction [Member]
Deferred income taxes [Abstract]
Tax credit carryforwards	212,000
Internal Revenue Service (IRS) [Member] | Domestic tax authority [Member]
Deferred income taxes [Abstract]
Current federal income tax refundable	59,000	$ 37,000
Net operating loss carryforwards	2,300,000
Internal Revenue Service (IRS) [Member] | R&D tax credit carryforwards [Member] | Domestic tax authority [Member]
Deferred income taxes [Abstract]
Tax credit carryforwards	7,400,000
Internal Revenue Service (IRS) [Member] | Alternative minimum tax credit carryforward [Member] | Domestic tax authority [Member]
Deferred income taxes [Abstract]
Tax credit carryforwards	$ 15,600,000