Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Natural gas and oil properties:
Properties subject to depletion	[1]	$ 9,716,557	$ 9,624,725
Unproved properties	[1]	940,064	943,246
Total	[1]	10,656,621	10,567,971
Accumulated depletion and depreciation	[1]	(2,871,827)	(2,590,398)
Natural gas and oil properties, successful efforts method, net	[1]	$ 7,784,794	$ 7,977,573

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