Earnings Per Share - Summary of Components Comprising Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Basic EPS Computation:
Numerator – Earnings available to common stockholders	$ 6,088	$ 5,351	$ 17,905	$ 14,677
Denominator – Weighted average number of common shares outstanding	7,637,448	7,559,136	7,615,096	7,538,860
Basic earnings per common share (in dollars per share)	$ 0.80	$ 0.71	$ 2.35	$ 1.95
Diluted EPS Computation:
Numerator – Earnings available to common stockholders	$ 6,088	$ 5,351	$ 17,905	$ 14,677
Denominator – Weighted average number of common shares outstanding	7,637,448	7,559,136	7,615,096	7,538,860
Dilutive effect of stock options (in shares)	3,348	4,218	3,434	4,350
Weighted average number of shares outstanding-diluted	7,640,796	7,563,354	7,618,530	7,543,210
Diluted earnings per common share (in dollars per share)	$ 0.80	$ 0.71	$ 2.35	$ 1.95