Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Standardized Measure of Discounted Future Net Cash Flows (Detail) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Extractive Industries [Abstract]
Future cash inflows		$ 21,290,873	$ 46,507,646
Future costs:
Production		(10,411,360)	(15,239,210)
Development	[1]	(2,213,582)	(4,275,693)
Future net cash flows before income taxes		8,665,931	26,992,743
Future income tax expense		(2,007,794)	(8,900,383)
Total future net cash flows before 10% discount		6,658,137	18,092,360
10% annual discount		(3,932,274)	(10,499,333)
Standardized measure of discounted future net cash flows		$ 2,725,863	$ 7,593,027	$ 5,862,231	$ 3,223,805

[1] 2015 includes $384.5 million of undiscounted future asset retirement costs estimated as of December 31, 2015, using current estimates of future abandonment costs.