Natural Gas Producing Activities (Unaudited) - Estimated Future Net Cash Flows from Natural Gas and Oil Reserves (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Future cash inflows	$ 51,423,920	$ 24,011,281	$ 17,619,037
Future production costs	(18,379,892)	(14,864,126)	(10,963,285)
Future development costs	(5,637,676)	(3,778,698)	(2,377,650)
Future income tax expenses	(5,811,125)	(1,753,067)	(1,333,989)
Future net cash flow	21,595,227	3,615,390	2,944,113
10% annual discount for estimated timing of cash flows	(12,593,293)	(2,626,636)	(1,966,559)
Standardized measure of discounted future net cash flows	$ 9,001,934	$ 988,754	$ 977,554	$ 4,810,028