Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income per consolidated statements of income	$ 2,951	$ 4,336	$ 7,248	$ 8,683
Net earnings allocated to participating securities	(15)	(17)	(34)	(34)
Net earnings allocated to common stock	2,936	4,319	7,214	8,649
Basic earnings per common share computation:
Net earnings allocated to common stock	2,936	4,319	7,214	8,649
Net earnings allocated to common stock	2,936	4,319	7,214	8,649
Weighted average common shares outstanding, including shares considered participating securities	12,510	12,440	12,502	12,421
Less: Average participating securities	(57)	(46)	(53)	(43)
Weighted average shares	12,453	12,394	12,449	12,378
Basic earnings per common share	$ 0.24	$ 0.35	$ 0.58	$ 0.70
Diluted earnings per common share computation:
Net earnings allocated to common stock	2,936	4,319	7,214	8,649
Weighted average common shares outstanding for basic earnings per common share	12,453	12,394	12,449	12,378
Add: Dilutive effects of assumed exercises of stock options	1	3	1	3
Weighted average shares and dilutive potential common shares	12,454	12,397	12,450	12,381
Diluted earnings per common share	$ 0.24	$ 0.35	$ 0.58	$ 0.70
Distributed Earnings Allocated to Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net earnings allocated to common stock	2,053	2,044	4,107	4,084
Basic earnings per common share computation:
Net earnings allocated to common stock	2,053	2,044	4,107	4,084
Net earnings allocated to common stock	2,053	2,044	4,107	4,084
Undistributed Earnings Allocated to Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net earnings allocated to common stock	883	2,275	3,107	4,565
Basic earnings per common share computation:
Net earnings allocated to common stock	883	2,275	3,107	4,565
Net earnings allocated to common stock	$ 883	$ 2,275	$ 3,107	$ 4,565