Income Taxes - Summary of Current and Deferred Components of Income Tax Expense (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current expense:
Federal									$ 3,981	$ 3,520	$ 3,181
State									498	416	315
Total current expense									4,479	3,936	3,496
Deferred (benefit) expense:
Federal									(3,179)	(2,564)	(1,833)
State									(434)	(365)	(271)
Total deferred benefit									(3,613)	(2,929)	(2,104)
Provision for income taxes	$ 121	$ 221	$ 231	$ 293	$ 116	$ 308	$ 295	$ 288	$ 866	$ 1,007	$ 1,392