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				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Derivative Instruments Gain Loss [Line Items]
Change in unrealized deferred hedging gains, tax effect			$ 33,403		$ 2,687		$ (21,780)
Change in unrealized deferred hedging (losses) gains, net of taxes			(52,246)	[1]	(4,203)	[1]	31,541	[1]
Realized gain (loss) reclassified from AOCI into revenue, income taxes	(10,967)	[2]	(23,972)	[2]	(38,343)	[2]	(76,805)	[2]
Realized gain on hedge derivative contract settlements reclassified into earnings from other comprehensive income, net of taxes	17,155	[2]	37,495	[2]	59,974	[2]	120,871	[2]
Swaps
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value			(33,311)		125		22,525
Realized Gain (Loss) Reclassified from OCI into Revenue	2,765	[2]	18,204	[2]	14,687	[2]	69,851	[2]
Put Option
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value			(994)				(1,908)
Realized Gain (Loss) Reclassified from OCI into Revenue			(682)	[2]			(998)	[2]
Collars
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value			(51,344)		(7,015)		32,704
Realized Gain (Loss) Reclassified from OCI into Revenue	$ 25,357	[2]	$ 43,945	[2]	$ 83,630	[2]	$ 128,823	[2]

[1] Amounts are net of income tax expense of $33,403 for the three months ended September 30, 2012 and income tax benefit of $21,780 for the nine months ended September 30, 2012. Amounts are net of income tax expense of $2,687 for the nine months ended September 30, 2013.

[2] 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.