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, 2015	Dec. 31, 2014	Dec. 31, 2013
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Sales and transfers of natural gas and oil produced – net	$ (757,789)	$ (1,479,242)	$ (1,060,566)
Net changes in prices, production and development costs	(5,566,232)	(1,525,944)	(292,533)
Extensions, discoveries and improved recovery, less related costs	264,735	2,300,923	1,509,002
Development costs incurred	971,186	1,023,075	1,319,135
Purchase of minerals in place – net	0	72,139	348,608
Sale of minerals in place – net	(43)	(146,476)	(252)
Revisions of previous quantity estimates	(1,541,419)	(222,195)	106,170
Accretion of discount	600,099	578,676	343,502
Net change in income taxes	2,424,200	(529,337)	(1,031,105)
Timing and other	(227,211)	787,130	554,159
Net (decrease) increase	(3,832,474)	858,749	1,796,120
Beginning of year	4,810,028	3,951,279	2,155,159
End of year	$ 977,554	$ 4,810,028	$ 3,951,279