Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Natural gas and oil properties:
Properties subject to depletion	$ 9,193,442	[1]	$ 8,225,859	[1]
Unproved properties	908,673	[1]	807,022	[1]
Total	10,102,115	[1]	9,032,881	[1]
Accumulated depreciation, depletion and amortization	(2,472,030)	[1]	(2,274,444)	[1]
Net capitalized costs	$ 7,630,085	[1]	$ 6,758,437	[1]

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