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Supplemental Gas Data (unaudited) - Change in Standardized Measure of Discounted Future Net Cash Flows (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Roll Forward]      
Balance at Beginning of Period $ 3,070,469 $ 4,655,457 $ 3,131,398
Net Changes in Sales Prices and Production Costs (819,247) (2,826,725) 1,732,229
Sales Net of Production Costs (719,441) (1,130,685) (995,630)
Net Change Due to Revisions in Quantity Estimates 322,820 (252,796) 307,030
Net Change Due to Extensions, Discoveries and Improved Recovery 268,196 654,027 534,052
Development Costs Incurred During the Period 434,273 739,874 844,081
Difference in Previously Estimated Development Costs Compared to Actual Costs Incurred During the Period (129,642) (323,922) (434,817)
Purchase of Reserves In-Place 0 0 209,630
Sales of Reserves In-Place 0 0 (434,103)
Changes in Estimated Future Development Costs (499,316) (24,469) (49,294)
Net Change in Future Income Taxes 138,404 409,797 (507,410)
Timing and Other 390,391 586,591 (69,087)
Accretion 178,829 583,320 387,378
Total Discounted Cash Flow at End of Period $ 2,635,736 $ 3,070,469 $ 4,655,457