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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, 2019
Dec. 31, 2018
Dec. 31, 2017
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 $ 4,655,457 $ 3,131,398 $ 955,117
Net Changes in Sales Prices and Production Costs (2,826,725) 1,732,229 1,983,475
Sales Net of Production Costs (1,130,685) (995,630) (831,131)
Net Change Due to Revisions in Quantity Estimates (252,796) 307,030 (145,496)
Net Change Due to Extensions, Discoveries and Improved Recovery 654,027 534,052 588,574
Development Costs Incurred During the Period 739,874 844,081 544,809
Difference in Previously Estimated Development Costs Compared to Actual Costs Incurred During the Period (323,922) (434,817) (129,427)
Purchase of Reserves In-Place 0 209,630 0
Sales of Reserves In-Place 0 (434,103) (55,277)
Changes in Estimated Future Development Costs (24,469) (49,294) (233,017)
Net Change in Future Income Taxes 409,797 (507,410) (404,582)
Timing and Other 586,591 (69,087) 712,764
Accretion 583,320 387,378 145,589
Total standardized measure of discounted net cash flows $ 3,070,469 $ 4,655,457 $ 3,131,398