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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic earnings per common share computation
Net income per consolidated statements of income	$ 5,508	$ 5,522	$ 5,602	$ 5,565	$ 6,743	$ 5,548	$ 5,617	$ 5,166	$ 22,197	$ 23,074	$ 16,679
Net earnings allocated to participating securities									(117)	(95)	(73)
Net earnings allocated to common stock									$ 22,080	$ 22,979	$ 16,606
Weighted average common shares outstanding, including shares considered participating securities									14,408	14,412	12,929
Less: Average participating securities									(70)	(53)	(50)
Weighted average shares									14,338	14,359	12,879
Basic earnings per common share	$ 0.38	$ 0.38	$ 0.39	$ 0.39	$ 0.47	$ 0.39	$ 0.39	$ 0.36	$ 1.54	$ 1.60	$ 1.29
Diluted earnings per common share computation
Net earnings allocated to common stock									$ 22,080	$ 22,979	$ 16,606
Weighted average common shares outstanding for basic earnings per common share									14,338	14,359	12,879
Add: Dilutive effects of assumed exercises of stock options									0	1	2
Weighted average shares and dilutive potential common shares									14,338	14,360	12,881
Diluted earnings per common share	$ 0.38	$ 0.38	$ 0.39	$ 0.39	$ 0.47	$ 0.39	$ 0.39	$ 0.36	$ 1.54	$ 1.60	$ 1.29
Distributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation
Net earnings allocated to common stock									$ 9,460	$ 9,476	$ 8,532
Undistributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation
Net earnings allocated to common stock									$ 12,620	$ 13,503	$ 8,074