Derivative Activities - Derivative Fair Value Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Derivatives, Fair Value [Line Items]
Change in fair value of derivatives that do not qualify for hedge accounting	$ (53,646)	[1]	$ 58,990	[1]	$ 30,075	[1]	$ 67,093	[1]
Realized gain (loss) on settlement - natural gas		[1],[2]	5,334	[1],[2]		[1],[2]	8,424	[1],[2]
Realized gain (loss) on settlement - oil	1,955	[1],[2]	285	[1],[2]	(1,899)	[1],[2]	(7,727)	[1],[2]
Realized gain (loss) on settlement - NGLs	14,682	[1],[2]	3,087	[1],[2]	20,442	[1],[2]	3,087	[1],[2]
Hedge ineffectiveness - realized	988		2,036		3,451		4,558
- unrealized	(4,707)		(3,971)		(5,061)		2,531
Derivative fair value (loss) income	$ (40,728)		$ 65,761		$ 47,008		$ 77,966

[1] Derivatives that do not qualify for hedge accounting.

[2] These amounts represent the realized gains and losses on settled derivatives that do not qualify 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 do not qualify for hedge accounting.