Income Taxes - Income Tax Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Current federal income tax	$ 30,381	$ 31,014	$ 29,832
Current state income tax	99	103	94
Deferred federal income tax expense (benefit)	673	320	(893)
Income tax expense	$ 31,153	$ 31,437	$ 29,033