Income Taxes - Components of Income Tax Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Components Of Income Tax Expense Benefit [Line Items]
Federal - Current
State - Current	7
Total current income tax expense	7
Federal - Deferred	82,841	42,809	38,153
State - Deferred	9,638	3,980	4,809
Total deferred income tax expense	92,479	46,789	42,962
Total income tax expense	$ 92,486	$ 46,789	$ 42,962