Derivative Activities - Effects of Cash Flow Hedges and Other Hedges on Accumulated Other Comprehensive Income (Detail) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Change in unrealized deferred hedging gains, tax effect,	$ 33,403,000		$ (34,195,000)		$ (21,780,000)		$ (41,609,000)
Change in Hedge Derivative Fair value, effects of Cash flow hedges on accumulated other comprehensive income (loss) total	(52,246,000)		56,993,000		31,541,000		74,530,000
Realized Gain Reclassified from AOCI into Revenue, Income taxes	(23,972,000)	[1]	(10,034,000)	[1]	(76,805,000)	[1]	(33,476,000)	[1]
Realized gain on hedge derivative contract settlements reclassified into earnings from other comprehensive income, net of taxes	37,495,000	[1]	16,724,000	[1]	120,871,000	[1]	55,791,000	[1]
Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Hedge Derivative Fair Value	(33,311)		15,739		22,525		17,854
Realized Gain Reclassified from OCI into Revenue	18,204	[1]			69,851	[1]
Put options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Hedge Derivative Fair Value	(994)				(1,908)
Realized Gain Reclassified from OCI into Revenue	(682)	[1]			(998)	[1]
Collars [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Hedge Derivative Fair Value	(51,344)		75,449		32,704		97,873
Realized Gain Reclassified from OCI into Revenue	43,945	[1]	26,758	[1]	128,823	[1]	80,660	[1]
Collars - discontinued operations [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in Hedge Derivative Fair Value							412
Realized Gain Reclassified from OCI into Revenue							$ 8,607	[1]

[1] For realized gains upon derivative contract settlement, the reduction in AOCI is offset by an increase in natural gas, NGLs and oil sales. For realized losses upon derivative contract settlement, the increase in AOCI is offset by a decrease in natural gas, NGLs and oil sales.