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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic earnings per common share computation
Net income per consolidated statements of income	$ 3,451	$ 7,246	$ 6,683	$ 6,480	$ 5,040	$ 6,416	$ 4,065	$ 5,019	$ 23,860	$ 20,540	$ 22,197
Net earnings allocated to participating securities									(135)	(129)	(117)
Net earnings allocated to common stock									$ 23,725	$ 20,411	$ 22,080
Weighted average common shares outstanding, including shares considered participating securities									15,212	14,457	14,408
Less: Average participating securities									(80)	(82)	(70)
Weighted average shares									15,132	14,375	14,338
Basic earnings per common share	$ 0.23	$ 0.47	$ 0.44	$ 0.43	$ 0.35	$ 0.44	$ 0.28	$ 0.35	$ 1.57	$ 1.42	$ 1.54
Diluted earnings per common share computation
Net earnings allocated to common stock									$ 23,725	$ 20,411	$ 22,080
Weighted average common shares outstanding for basic earnings per common share									15,132	14,375	14,338
Add: Dilutive effects of assumed exercises of stock options									0	0	0
Weighted average shares and dilutive potential common shares									15,132	14,375	14,338
Diluted earnings per common share	$ 0.23	$ 0.47	$ 0.44	$ 0.43	$ 0.35	$ 0.44	$ 0.28	$ 0.35	$ 1.57	$ 1.42	$ 1.54
Distributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation
Net earnings allocated to common stock									$ 10,030	$ 9,480	$ 9,460
Undistributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation
Net earnings allocated to common stock									$ 13,695	$ 10,931	$ 12,620