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, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Extractive Industries [Abstract]
Future cash inflows		$ 27,413,864	$ 21,290,873
Future costs:
Production		(14,465,059)	(10,411,360)
Development	[1]	(2,647,801)	(2,213,582)
Future net cash flows before income taxes		10,301,004	8,665,931
Future income tax expense		(1,946,259)	(2,007,794)
Total future net cash flows before 10% discount		8,354,745	6,658,137
10% annual discount		(4,902,816)	(3,932,274)
Standardized measure of discounted future net cash flows		$ 3,451,929	$ 2,725,863	$ 7,593,027	$ 5,862,231

[1] 2016 includes $405.3 million of undiscounted future asset retirement costs estimated as of December 31, 2016, using current estimates of future abandonment costs.