Derivative Activities - Effects of Cash Flow Hedges and Other Hedges on Accumulated Other Comprehensive 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
Derivative Instruments Gain Loss [Line Items]
Change in unrealized deferred hedging gains, tax effect			$ (3,077)		$ 2,687		$ (55,183)
Realized Gain Reclassified from AOCI into Revenue, Income taxes	(12,252)	[1]	(30,647)	[1]	(27,376)	[1]	(52,834)	[1]
Change in unrealized deferred hedging (losses) gains, net of taxes			4,813	[2]	(4,203)	[2]	83,787	[2]
Realized gain on hedge derivative contract settlements reclassified into earnings from other comprehensive income, net of taxes	19,163	[1]	47,934	[1]	42,818	[1]	83,376	[1]
Swap
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value			19,665		125		55,836
Realized Gain (Loss) Reclassified from OCI into Revenue	3,875	[1]	32,335	[1]	11,922	[1]	51,647	[1]
Put Option
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value			648				(914)
Realized Gain (Loss) Reclassified from OCI into Revenue			(315)	[1]			(315)	[1]
Collars
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value			(12,423)		(7,015)		84,048
Realized Gain (Loss) Reclassified from OCI into Revenue	$ 27,540	[1]	$ 46,561	[1]	$ 58,272	[1]	$ 84,878	[1]

[1] For realized gains upon derivative contract settlement, the reduction in AOCI is offset by an increase in revenues, NGLs and oil sales. For realized losses upon derivative contract settlement, the increase in AOCI is offset by a decrease in revenues. See additional discussion above regarding the discontinuance of hedge accounting.

[2] Presented net of income tax benefit of $3,077 for the three months ended June 30, 2012 and $55,184 for the six months ended June 30, 2012. Presented net of income tax expense of $2,687 for the six months ended June 30, 2013.