Summary of Significant Accounting Policies - Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance, Gross	$ 137,555		$ 254,678		$ 111,062
Contract settlements reclassified to income, Gross	(120,443)		(236,305)		(132,201)
Change in unrealized deferred hedging gains, Gross	(6,890)		119,182		275,817
Ending Balance, Gross	10,222		137,555		254,678
Beginning Balance, Tax Effect	(53,646)		(98,051)		(43,592)
Contract settlements reclassified to income, Tax Effect	(46,973)	[1]	(91,871)	[1]	50,005
Change in unrealized deferred hedging (losses) gains, tax effect	2,687		(47,466)		(104,464)
Ending Balance, Tax Effect	(3,986)		(53,646)		(98,051)
Beginning Balance, Net of Tax	83,909		156,627		67,470
Contract settlements reclassified to income, Net of Tax	(73,470)		(144,434)		(82,196)
Change in unrealized deferred hedging (losses) gains, net of taxes	(4,203)	[2]	71,716	[2]	171,353	[2]
Ending Balance, Net of Tax	$ 6,236		$ 83,909		$ 156,627

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

[2] Amounts are net of income tax benefit of $2,687 for the year ended December 31, 2013 compared to income tax expense of $47,466 for the year ended December 31, 2012 and income tax expense of $104,464 for the twelve months ended December 31, 2011.