Derivative Activities - Derivative Fair Value Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Derivatives Fair Value [Line Items]
Change in fair value of derivatives that did not qualify or were not designated for hedge accounting	$ (34,219)	[1]	$ (53,646)	[1]	$ 28,350	[1]	$ 30,075	[1]
Realized gain (loss) on settlement–natural gas	5,815	[1],[2]			(17,913)	[1],[2]
Realized (loss) gain on settlement–oil	(8,005)	[1],[2]	1,955	[1],[2]	(8,218)	[1],[2]	(1,899)	[1],[2]
Realized (loss) gain on settlement–NGLs	(3,907)	[1],[2]	14,682	[1],[2]	(1,759)	[1],[2]	20,442	[1],[2]
Hedge ineffectiveness-realized	(854)		988		(445)		3,451
Hedge ineffectiveness-unrealized	815		(4,707)		(2,485)		(5,061)
Derivative fair value (loss) income	$ (40,355)		$ (40,728)		$ (2,470)		$ 47,008

[1] Derivatives that did not qualify or were not designated for hedge accounting. Change in fair value of derivatives line also includes gains of $3.1 million in third quarter 2013 and gains of $25.5 million in the first nine months 2013 related to discontinuance of hedge accounting.

[2] These amounts represent the realized gains and losses on settled derivatives that did not qualify or were not designated for hedge accounting, which before settlement are included in the category in this same table referred to as change in fair value of derivatives that did not qualify or were not designated for hedge accounting.