Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015
Natural gas and oil properties:
Properties subject to depletion	[1]	$ 8,030,190	$ 8,047,181
Unproved properties	[1]	935,189	949,155
Total	[1]	8,965,379	8,996,336
Accumulated depletion and depreciation	[1]	(2,748,635)	(2,635,031)
Natural gas and oil properties, successful efforts method, net	[1]	$ 6,216,744	$ 6,361,305

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