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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic earnings per common share computation
Net income per consolidated statements of income	$ 5,040	$ 6,416	$ 4,065	$ 5,019	$ 5,508	$ 5,522	$ 5,602	$ 5,565	$ 20,540	$ 22,197	$ 23,074
Net earnings allocated to participating securities									(129)	(117)	(95)
Net earnings allocated to common stock									$ 20,411	$ 22,080	$ 22,979
Weighted average common shares outstanding, including shares considered participating securities									14,457	14,408	14,412
Less: Average participating securities									(82)	(70)	(53)
Weighted average shares									14,375	14,338	14,359
Basic earnings per common share	$ 0.35	$ 0.44	$ 0.28	$ 0.35	$ 0.38	$ 0.38	$ 0.39	$ 0.39	$ 1.42	$ 1.54	$ 1.60
Diluted earnings per common share computation
Net earnings allocated to common stock									$ 20,411	$ 22,080	$ 22,979
Weighted average common shares outstanding for basic earnings per common share									14,375	14,338	14,359
Add: Dilutive effects of assumed exercises of stock options									0	0	1
Weighted average shares and dilutive potential common shares									14,375	14,338	14,360
Diluted earnings per common share	$ 0.35	$ 0.44	$ 0.28	$ 0.35	$ 0.38	$ 0.38	$ 0.39	$ 0.39	$ 1.42	$ 1.54	$ 1.60
Distributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation
Net earnings allocated to common stock									$ 9,480	$ 9,460	$ 9,476
Undistributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation
Net earnings allocated to common stock									$ 10,931	$ 12,620	$ 13,503