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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic earnings per common share computation
Net income per consolidated statements of income	$ 8,945	$ 9,236	$ 8,441	$ 7,097	$ 3,451	$ 7,246	$ 6,683	$ 6,480	$ 33,719	$ 23,860	$ 20,540
Net earnings allocated to participating securities									(150)	(135)	(129)
Net earnings allocated to common stock									$ 33,569	$ 23,725	$ 20,411
Weighted average common shares outstanding, including shares considered participating securities (in shares)									15,274	15,212	14,457
Less: Average participating securities (in shares)									(64)	(80)	(82)
Weighted average shares (in shares)									15,210	15,132	14,375
Basic earnings per common share (in dollars per share)	$ 0.59	$ 0.60	$ 0.55	$ 0.46	$ 0.23	$ 0.47	$ 0.44	$ 0.43	$ 2.21	$ 1.57	$ 1.42
Diluted earnings per common share computation
Net earnings allocated to common stock									$ 33,569	$ 23,725	$ 20,411
Weighted average common shares outstanding for basic earnings per common share (in shares)									15,210	15,132	14,375
Add: Dilutive effects of assumed exercises of stock options (in shares)									0	0	0
Weighted average shares and dilutive potential common shares (in shares)									15,210	15,132	14,375
Diluted earnings per common share (in dollars per share)	$ 0.59	$ 0.60	$ 0.55	$ 0.46	$ 0.23	$ 0.47	$ 0.44	$ 0.43	$ 2.21	$ 1.57	$ 1.42
Distributed earnings allocated to common stock
Basic earnings per common share computation
Net earnings allocated to common stock									$ 10,186	$ 10,030	$ 9,480
Undistributed earnings allocated to common stock
Basic earnings per common share computation
Net earnings allocated to common stock									$ 23,383	$ 13,695	$ 10,931