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, 2015	Dec. 31, 2014	Dec. 31, 2013
Natural gas and oil properties:
Properties subject to depletion	[1]	$ 8,047,181	$ 9,624,725	$ 8,225,859
Unproved properties	[1]	949,155	943,246	807,022
Total	[1]	8,996,336	10,567,971	9,032,881
Accumulated depletion and depreciation	[1]	(2,635,031)	(2,590,398)	(2,274,444)
Natural gas and oil properties, successful efforts method, net	[1]	$ 6,361,305	$ 7,977,573	$ 6,758,437

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