Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014
Natural gas and oil properties:
Properties subject to depletion	[1]	$ 10,019,744	$ 9,624,725
Unproved properties	[1]	946,416	943,246
Total	[1]	10,966,160	10,567,971
Accumulated depletion and depreciation	[1]	(2,725,802)	(2,590,398)
Natural gas and oil properties, successful efforts method, net	[1]	$ 8,240,358	$ 7,977,573

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