Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of the provision for income taxes
Current Federal Tax Expense (Benefit)	$ 0	$ 3,826	$ 0
Current State and Local Tax Expense (Benefit)	(449)	(1,618)	(1,662)
Deferred portion of income tax (expense) benefit	192,540	41,835	(141,708)
Income tax (expense) benefit	$ 192,091	$ 44,043	$ (143,370)
Effective tax rate (as a percent)	20.10%	19.10%	22.00%
Deferred income taxes [Abstract]
Deferred tax liabilities, oil and gas properties	$ 83,816	$ 224,686
Deferred tax liabilities, derivatives assets	0	4,646
Deferred tax liabilities, other	10,054	12,361
Total deferred tax liabilities	93,870	241,693
Derivative tax assets, derivative liabilities	36,311	0
Derivative tax assets, debt discount and deferred financing costs	23,925	0
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Asset Retirement Obligations	18,424	19,658
Deferred tax assets, credit carryover	7,543	11,270
Deferred tax assets, pension	7,183	5,971
Deferred tax assets, federal and state tax net operating loss carryovers	3,898	4,172
Deferred tax assets, stock compensation	2,701	3,503
Deferred tax assets, other liabilities	7,273	10,803
Total deferred tax assets	107,258	55,377
Deferred tax assets, valuation allowance	(13,388)	(3,070)
Net deferred tax assets	93,870	52,307
Total net deferred tax liabilities	0	189,386
State and local jurisdiction [Member]
Deferred income taxes [Abstract]
Current state income tax payable	853	1,404
Net operating loss carryforwards	4,900
Internal Revenue Service (IRS) [Member] | Domestic tax authority [Member]
Deferred income taxes [Abstract]
Current federal income tax refundable	0	$ 3,885
Internal Revenue Service (IRS) [Member] | Domestic tax authority [Member] | Research and development tax credit carryforward [Member]
Deferred income taxes [Abstract]
Deferred tax assets, credit carryover	$ 7,500