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, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Extractive Industries [Abstract]
Future cash inflows		$ 43,500,054	$ 27,413,864
Future costs:
Production		(18,958,695)	(14,465,059)
Development	[1]	(3,072,688)	(2,647,801)
Future net cash flows before income taxes		21,468,671	10,301,004
Future income tax expense		(3,989,459)	(1,946,259)
Total future net cash flows before 10% discount		17,479,212	8,354,745
10% annual discount		(10,313,998)	(4,902,816)
Standardized measure of discounted future net cash flows		$ 7,165,214	$ 3,451,929	$ 2,725,863	$ 7,593,027

[1] 2017 includes $430.6 million of undiscounted future asset retirement costs estimated as of December 31, 2017, using current estimates of future abandonment costs.