Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Natural gas and oil properties:
Properties subject to depletion	[1]	$ 10,172,411	$ 9,462,350
Unproved properties	[1]	2,888,979	2,923,803
Total	[1]	13,061,390	12,386,153
Accumulated depletion and depreciation	[1]	(3,492,614)	(3,129,816)
Natural gas and oil properties, successful efforts method, net	[1]	$ 9,568,776	$ 9,256,337

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