Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015
Natural gas and oil properties:
Properties subject to depletion	[1]	$ 8,074,869	$ 8,047,181
Unproved properties	[1]	930,142	949,155
Total	[1]	9,005,011	8,996,336
Accumulated depletion and depreciation	[1]	(2,864,358)	(2,635,031)
Natural gas and oil properties, successful efforts method, net	[1]	$ 6,140,653	$ 6,361,305

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