Derivative Activities - Derivative Fair Value Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Derivatives Fair Value [Line Items]
Change in fair value of derivatives that do not qualify or are not designated for hedge accounting	$ 159,371	[1]	$ 135,777	[1]	$ 62,569	[1]	$ 83,721	[1]
Realized loss on settlement–natural gas	(24,543)	[1],[2]			(23,728)	[1],[2]
Realized gain (loss) on settlement–oil	(111)	[1],[2]	768	[1],[2]	(213)	[1],[2]	(3,854)	[1],[2]
Realized gain on settlement–NGLs	3,043	[1],[2]	10,152	[1],[2]	2,148	[1],[2]	5,760	[1],[2]
Hedge ineffectiveness-realized	(155)		1,278		409		2,463
Hedge ineffectiveness-unrealized	155		594		(3,300)		(354)
Derivative fair value income	$ 137,760		$ 148,569		$ 37,885		$ 87,736

[1] Derivatives that did not qualify or were not designated for hedge accounting. Change in fair value of derivatives line also includes gains of $103.8 million in second quarter 2013 and gains of $22.4 million in the first six 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.