Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Natural gas and oil properties:
Properties subject to depletion	[1]	$ 10,572,453	$ 9,462,350	$ 8,047,181
Unproved properties	[1]	2,644,000	2,923,803	949,155
Total	[1]	13,216,453	12,386,153	8,996,336
Accumulated depletion and depreciation	[1]	(3,649,716)	(3,129,816)	(2,635,031)
Natural gas and oil properties, successful efforts method, net	[1]	$ 9,566,737	$ 9,256,337	$ 6,361,305

[1] Includes capitalized asset retirement costs and the associated accumulated amortization.