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, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value							$ (4,203)
Realized Gain (Loss) Reclassified from OCI into Revenue	616,615		442,038		1,839,481		1,434,633
Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments Gain Loss [Line Items]
Realized Gain (Loss) Reclassified from OCI into Revenue	2,172	[1]	17,155	[1]	6,458	[1]	59,974	[1]
Tax Effect
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value							2,687
Tax Effect | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments Gain Loss [Line Items]
Realized Gain (Loss) Reclassified from OCI into Revenue	(1,332)	[1]	(10,967)	[1]	(4,122)	[1]	(38,343)	[1]
Swaps | Before Tax
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value							125
Swaps | Before Tax | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments Gain Loss [Line Items]
Realized Gain (Loss) Reclassified from OCI into Revenue	1,255	[1]	2,765	[1]	3,144	[1]	14,687	[1]
Collars | Before Tax
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value							(7,015)
Collars | Before Tax | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments Gain Loss [Line Items]
Realized Gain (Loss) Reclassified from OCI into Revenue	$ 2,249	[1]	$ 25,357	[1]	$ 7,436	[1]	$ 83,630	[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 revenues. See additional discussion above regarding the discontinuance of hedge accounting.