Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic earnings per common share computation
Net income per consolidated statements of income	$ 10,484	$ 10,357	$ 9,767	$ 9,473	$ 8,945	$ 9,236	$ 8,441	$ 7,097	$ 40,081	$ 33,719	$ 23,860
Net earnings allocated to participating securities									(147)	(150)	(135)
Net earnings allocated to common stock									$ 39,934	$ 33,569	$ 23,725
Weighted average common shares outstanding, including shares considered participating securities (in shares)									15,219	15,274	15,212
Less: Average participating securities (in shares)									(55)	(64)	(80)
Weighted average shares (in shares)									15,164	15,210	15,132
Basic earnings per common share (in dollars per share)	$ 0.69	$ 0.68	$ 0.64	$ 0.62	$ 0.59	$ 0.60	$ 0.55	$ 0.46	$ 2.63	$ 2.21	$ 1.57
Diluted earnings per common share computation
Net earnings allocated to common stock									$ 39,934	$ 33,569	$ 23,725
Weighted average common shares outstanding for basic earnings per common share (in shares)									15,164	15,210	15,132
Add: Dilutive effects of assumed exercises of stock options (in shares)									0	0	0
Weighted average shares and dilutive potential common shares (in shares)									15,164	15,210	15,132
Diluted earnings per common share (in dollars per share)	$ 0.69	$ 0.68	$ 0.64	$ 0.62	$ 0.59	$ 0.60	$ 0.55	$ 0.46	$ 2.63	$ 2.21	$ 1.57
Distributed earnings allocated to common stock
Basic earnings per common share computation
Net earnings allocated to common stock									$ 10,317	$ 10,186	$ 10,030
Undistributed earnings allocated to common stock
Basic earnings per common share computation
Net earnings allocated to common stock									$ 29,617	$ 23,383	$ 13,695