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								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value											$ (4,203)
Realized Gain Reclassified from OCI Into Revenue	872,214	616,615	765,524	457,342	428,086	442,038	673,356	319,239	2,711,695		1,862,719		1,457,704
Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments Gain Loss [Line Items]
Realized Gain Reclassified from OCI Into Revenue									6,236	[1]	73,470	[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 Reclassified from OCI Into Revenue									(3,986)	[1]	(46,973)	[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 Reclassified from OCI Into Revenue									4,544	[1]	15,171	[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 Reclassified from OCI Into Revenue									$ 5,678	[1]	$ 105,272	[1]

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