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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic earnings per common share computation:
Net income per consolidated statements of income	$ 7,246	$ 6,416	$ 20,409	$ 15,500
Net earnings allocated to participating securities	(40)	(40)	(120)	(101)
Net earnings allocated to common stock	$ 7,206	$ 6,376	$ 20,289	$ 15,399
Weighted average common shares outstanding, including shares considered participating securities	15,285	14,464	15,188	14,453
Less: Average participating securities	(78)	(82)	(84)	(85)
Weighted average shares	15,207	14,382	15,104	14,368
Basic earnings per common share	$ 0.47	$ 0.44	$ 1.34	$ 1.07
Diluted earnings per common share computation:
Net earnings allocated to common stock	$ 7,206	$ 6,376	$ 20,289	$ 15,399
Weighted average shares and dilutive potential common shares	15,207	14,382	15,104	14,368
Diluted earnings per common share	$ 0.47	$ 0.44	$ 1.34	$ 1.07
Distributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 2,506	$ 2,370	$ 7,521	$ 7,107
Undistributed Earnings Allocated to Common Stock [Member]
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 4,700	$ 4,006	$ 12,768	$ 8,292