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, 2017	Dec. 31, 2016	Dec. 31, 2015
Revisions of previous estimates
Changes in prices and production costs	$ 2,615,825	$ (212,867)	$ (7,231,629)
Revisions in quantities	445,667	96,615	(868,886)
Changes in future development and abandonment costs	(497,400)	(314,864)	359,540
Net change in income taxes	(706,531)	27,842	2,173,904
Accretion of discount	372,743	302,920	1,007,027
Purchases of reserves in place	6,173	488,959	0
Additions to proved reserves from extensions, discoveries and improved recovery	2,128,135	541,095	486,478
Natural gas, NGLs and oil sales, net of production costs	(1,237,970)	(509,174)	(522,682)
Development costs incurred during the period	885,803	435,928	1,033,539
Sales of reserves in place	(32,946)	(65,538)	(1,050,237)
Timing and other	(266,214)	(64,850)	(254,218)
Net change for the year	3,713,285	726,066	(4,867,164)
Beginning of year	3,451,929	2,725,863	7,593,027
End of year	$ 7,165,214	$ 3,451,929	$ 2,725,863