Stock-Based Compensation - Components of Stock-Based Compensation (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Total stock-based compensation expense	$ 2,322	$ 1,798	$ 1,306
Tax benefit of stock-based compensation expense	(892)	(671)	(472)
After-tax effect of stock based compensation	1,430	1,127	834
Stock options
Total stock-based compensation expense	236	(245)	(66)
Restricted stock units
Total stock-based compensation expense	2,086	2,043	1,314
Deferred directors fees
Total stock-based compensation expense	$ 0	$ 0	$ 58