Capitalized Costs and Accumulated Depreciation, Depletion and Amortization - Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Natural gas and oil properties:
Properties subject to depletion	$ 7,219,474	[1]	$ 6,035,429	[1]
Unproved properties	781,371	[1]	748,598	[1]
Total	8,000,845	[1]	6,784,027	[1]
Accumulated depreciation, depletion and amortization	(1,942,698)	[1]	(1,626,461)	[1]
Net capitalized costs	$ 6,058,147	[1]	$ 5,157,566	[1]

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