Derivative Activities - Derivative Fair Value (Loss) Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Derivatives Fair Value [Line Items]
Change in fair value of derivatives that do not qualify for hedge accounting	$ (96,802)	[1]	$ (52,056)	[1]
Realized gain (loss) on settlement–natural gas	815	[1],[2]
Realized gain (loss) on settlement–oil	(102)	[1],[2]	(4,622)	[1],[2]
Realized gain (loss) on settlement–NGLs	(895)	[1],[2]	(4,392)	[1],[2]
Hedge ineffectiveness–realized	564		1,185
Hedge ineffectiveness-unrealized	(3,455)		(948)
Derivative fair value loss	$ (99,875)		$ (60,833)

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

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