Summary of Significant Accounting Policies - Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013		Dec. 31, 2012
Disclosure Summary Of Significant Accounting Policies Accumulated Other Comprehensive Income [Abstract]
Beginning Balance, Gross	$ 10,222	$ 137,555		$ 254,678
Contract settlements reclassified to income, Gross	(10,222)	(120,443)		(236,305)
Change in unrealized deferred hedging gains (losses), Gross		(6,890)		119,182
Ending Balance, Gross		10,222		137,555
Beginning Balance, Tax Effect	(3,986)	(53,646)		(98,051)
Contract settlements reclassified to income, Tax Effect	3,986	46,973		91,871
Change in unrealized deferred hedging (losses) gains, tax effect		2,687		(47,466)
Ending Balance, Tax Effect		(3,986)		(53,646)
Beginning Balance, Net of Tax	6,236	83,909		156,627
Contract settlements reclassified to income, Net of Tax	(6,236)	(73,470)		(144,434)
Change in unrealized deferred hedging (losses) gains, net of taxes		(4,203)	[1]	71,716	[1]
Ending Balance, Net of Tax		$ 6,236		$ 83,909

[1] 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.