Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic earnings per common share computation:
Net income per consolidated statements of income	$ 7,097	$ 6,480
Net earnings allocated to participating securities	(34)	(39)
Net earnings allocated to common stock	$ 7,063	$ 6,441
Weighted average common shares outstanding, including shares considered participating securities	15,273	14,979
Less: Average participating securities	(72)	(86)
Weighted average shares	15,201	14,893
Basic earnings per common share	$ 0.46	$ 0.43
Diluted earnings per common share computation:
Net earnings allocated to common stock	$ 7,063	$ 6,441
Weighted average common shares outstanding for basic earnings per common share	15,201	14,893
Add: Dilutive effects of assumed exercises of stock options	0	0
Weighted average shares and dilutive potential common shares	15,201	14,893
Diluted earnings per common share	$ 0.46	$ 0.43
Distributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 2,509	$ 2,508
Undistributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 4,554	$ 3,933