Natural Gas Producing Activities (Unaudited) - Summary of Changes in the Standardized Measure of Discounted 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]
Sales and transfers of natural gas and oil produced – net	$ (2,802,742)	$ (1,305,186)	$ (540,636)
Net changes in prices, production and development costs	2,949,606	2,236,183	(1,129,026)
Extensions, discoveries and improved recovery, less related costs	1,616,653	1,269,712	590,885
Development costs incurred	1,630,506	712,635	402,891
Purchase of minerals in place – net	0	5,357,921	592,078
Sale of minerals in place – net	(849,162)	(284)	0
Revisions of previous quantity estimates	(811,576)	(297,437)	(60,959)
Accretion of discount	834,026	115,437	122,674
Net change in income taxes	(289,549)	(1,477,603)	(91,823)
Timing and other	332,202	1,401,802	125,116
Net increase (decrease)	2,609,964	8,013,180	11,200
Beginning of year	9,001,934	988,754	977,554
End of year	$ 11,611,898	$ 9,001,934	$ 988,754