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, 2013		Dec. 31, 2012
Natural gas and oil properties:
Properties subject to depletion	$ 7,919,627	[1]	$ 7,368,308	[1]
Unproved properties	748,055	[1]	743,467	[1]
Total	8,667,682	[1]	8,111,775	[1]
Accumulated depreciation, depletion and amortization	(2,160,378)	[1]	(2,015,591)	[1]
Net capitalized costs	$ 6,507,304	[1]	$ 6,096,184	[1]

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