Supplemental Gas Data Change in Standardized Measure of Discounted Future Net Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	$ 3,131,398	$ 955,117	$ 1,019,304
Net changes in sales prices and production costs	1,732,229	1,983,475	(172,812)
Sales net of production costs	(995,630)	(831,131)	(150,819)
Net change due to revisions in quantity estimates	307,030	(145,496)	(35,502)
Net change due to extensions, discoveries and improved recovery	534,052	588,574	(54,628)
Development costs incurred during the period	844,081	544,809	138,813
Difference in previously estimated development costs compared to actual costs incurred during the period	(434,817)	(129,427)	(39,821)
Purchase of Reserves In-Place	209,630	0	238,819
Sales of Reserves In-Place	(434,103)	(55,277)	(137,998)
Changes in estimated future development costs	(49,294)	(233,017)	(158,000)
Net change in future income taxes	(507,410)	(404,582)	36,513
Timing and Other	(69,087)	712,764	125,529
Accretion	387,378	145,589	145,719
Total standardized measure of discounted net cash flows	$ 4,655,457	$ 3,131,398	$ 955,117