Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Basic earnings per common share computation:
Net income per consolidated statements of income	$ 5,522	$ 5,548	$ 16,689	$ 16,331
Net earnings allocated to participating securities	(30)	(23)	(92)	(70)
Net earnings allocated to common stock	$ 5,492	$ 5,525	$ 16,597	$ 16,261
Weighted average common shares outstanding, including shares considered participating securities	14,406	14,368	14,408	14,421
Less: Average participating securities	(71)	(53)	(72)	(55)
Weighted average shares	14,335	14,315	14,336	14,366
Basic earnings per common share	$ 0.38	$ 0.39	$ 1.16	$ 1.13
Diluted earnings per common share computation:
Net earnings allocated to common stock	$ 5,492	$ 5,525	$ 16,597	$ 16,261
Weighted average common shares outstanding for basic earnings per common share	14,335	14,315	14,336	14,366
Add: Dilutive effects of assumed exercises of stock options	0	1	0	1
Weighted average shares and dilutive potential common shares	14,335	14,316	14,336	14,367
Diluted earnings per common share	$ 0.38	$ 0.39	$ 1.16	$ 1.13
Distributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 2,363	$ 2,359	$ 7,095	$ 7,109
Undistributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 3,129	$ 3,166	$ 9,502	$ 9,152