Natural Gas Producing Activities (Unaudited) - Estimated Future Net Cash Flows from Natural Gas and Oil Reserves (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Future cash inflows	$ 24,011,281	$ 17,619,037	$ 42,352,358
Future production costs	(14,864,126)	(10,963,285)	(16,791,623)
Future development costs	(3,778,698)	(2,377,650)	(5,052,195)
Future income tax expenses	(1,753,067)	(1,333,989)	(7,718,406)
Future net cash flow	3,615,390	2,944,113	12,790,134
10% annual discount for estimated timing of cash flows	(2,626,636)	(1,966,559)	(7,980,106)
Standardized measure of discounted future net cash flows	$ 988,754	$ 977,554	$ 4,810,028	$ 3,951,279