Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of the provision for income taxes
Federal	$ (3,826)	$ 0	$ 5,698
State	1,618	1,662	3,398
Deferred portion of income tax expense (benefit)	(41,835)	141,708	(192,066)
Income tax expense (benefit)	$ (44,043)	$ 143,370	$ (182,970)
Effective tax rate (as a percent)	19.10%	22.00%	53.20%
Deferred income taxes [Abstract]
Deferred tax liabilities, oil and gas properties	$ 205,028	$ 218,094
Deferred tax liabilities, derivatives assets	4,646	35,247
Deferred tax liabilities, other	12,361	4,812
Total deferred tax liabilities	222,035	258,153
Deferred tax assets, credit carryover	11,270	22,554
Deferred tax assets, pension	5,971	6,427
Deferred tax assets, federal and state tax net operating loss carryovers	4,172	4,217
Deferred tax assets, stock compensation	3,503	3,263
Deferred tax assets, other liabilities	10,803	1,497
Total deferred tax assets	35,719	37,958
Deferred tax assets, valuation allowance	(3,070)	(3,083)
Net deferred tax assets	32,649	34,875
Total net deferred tax liabilities	189,386	223,278
State and local jurisdiction [Member]
Deferred income taxes [Abstract]
Current state income tax payable	1,404	1,331
Net operating loss carryforwards	77,800
Internal Revenue Service (IRS) [Member] | Domestic tax authority [Member]
Deferred income taxes [Abstract]
Current federal income tax refundable	3,885	$ 59
Net operating loss carryforwards	3,300
Internal Revenue Service (IRS) [Member] | Research and development tax credit carryforward [Member] | Domestic tax authority [Member]
Deferred income taxes [Abstract]
Tax credit carryforwards	7,400
Internal Revenue Service (IRS) [Member] | Alternative minimum tax credit carryforward [Member] | Domestic tax authority [Member]
Deferred income taxes [Abstract]
Tax credit carryforwards	$ 4,300