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
	Mar. 31, 2014		Mar. 31, 2013
Derivative Instruments Gain Loss [Line Items]
Change in unrealized deferred hedging gains, tax effect			$ 2,687
Change in Hedge Derivative Fair value, effects of Cash flow hedges on accumulated other comprehensive income (loss) total			(4,203)	[1]
Realized Gain Reclassified from AOCI into Revenue, Income taxes	(924)	[2]	(15,124)	[2]
Realized gain on hedge derivative contract settlements reclassified into earnings from other comprehensive income, net of taxes	1,240	[2]	23,655	[2]
Swaps
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value			125
Realized Gain Reclassified from OCI into Revenue	836	[2]	8,047	[2]
Collars
Derivative Instruments Gain Loss [Line Items]
Change in Hedge Derivative Fair Value			(7,015)
Realized Gain Reclassified from OCI into Revenue	$ 1,328	[2]	$ 30,732	[2]

[1] Amounts are net of income tax benefit of $2,687 for the three months ended March 31, 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.