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, 2016	Dec. 31, 2015	Dec. 31, 2014
Natural gas and oil properties:
Properties subject to depletion	[1]	$ 9,462,350	$ 8,047,181	$ 9,624,725
Unproved properties	[1]	2,923,803	949,155	943,246
Total	[1]	12,386,153	8,996,336	10,567,971
Accumulated depletion and depreciation	[1]	(3,129,816)	(2,635,031)	(2,590,398)
Natural gas and oil properties, successful efforts method, net	[1]	$ 9,256,337	$ 6,361,305	$ 7,977,573

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