Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Changes in Discounted Future Net Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revisions of previous estimates
Changes in prices and production costs	$ 5,069	$ 2,172,704	$ (2,498,616)
Revisions in quantities	102,760	513,168	88,190
Changes in future development and abandonment costs	(407,688)	(275,468)	(354,766)
Net change in income taxes	(441,935)	(1,299,227)	832,830
Accretion of discount	789,754	395,989	608,381
Purchases of reserves in place	297,358
Additions to proved reserves from extensions, discoveries and improved recovery	2,713,999	1,981,054	1,429,340
Natural gas, NGLs and oil sales, net of production costs	(1,391,663)	(1,286,103)	(976,224)
Development costs incurred during the period	755,384	462,862	562,329
Sales of reserves in place	(249,055)	(162,463)	(120,637)
Timing and other	(443,187)	135,910	(861,919)
Net change for the year	1,730,796	2,638,426	(1,291,092)
Beginning of year	5,862,231	3,223,805	4,514,897
End of year	$ 7,593,027	$ 5,862,231	$ 3,223,805