Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Changes in Discounted Future Net Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revisions of previous estimates
Changes in prices and production costs	$ (212,867)	$ (7,231,629)	$ 5,069
Revisions in quantities	96,615	(868,886)	102,760
Changes in future development and abandonment costs	(314,864)	359,540	(407,688)
Net change in income taxes	27,842	2,173,904	(441,935)
Accretion of discount	302,920	1,007,027	789,754
Purchases of reserves in place	488,959	0	297,358
Additions to proved reserves from extensions, discoveries and improved recovery	541,095	486,478	2,713,999
Natural gas, NGLs and oil sales, net of production costs	(509,174)	(522,682)	(1,391,663)
Development costs incurred during the period	435,928	1,033,539	755,384
Sales of reserves in place	(65,538)	(1,050,237)	(249,055)
Timing and other	(64,850)	(254,218)	(443,187)
Net change for the year	726,066	(4,867,164)	1,730,796
Beginning of year	2,725,863	7,593,027	5,862,231
End of year	$ 3,451,929	$ 2,725,863	$ 7,593,027