Income Taxes - Income Tax Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Expenses [Line Items]
Current federal income tax	$ 23,605	$ 21,727	$ 19,625
Current state income tax	13	84	51
Deferred federal income tax expense (benefit)	(1,517)	2,005	1,089
Income tax expense	$ 25,135	$ 23,816	$ 20,765