Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic earnings per common share computation:
Net income per consolidated statements of income	$ 9,767	$ 8,441	$ 19,240	$ 15,538
Net earnings allocated to participating securities	(40)	(39)	(78)	(73)
Net earnings allocated to common stock	$ 9,727	$ 8,402	$ 19,162	$ 15,465
Weighted average common shares outstanding, including shares considered participating securities (in shares)	15,230,000	15,286,000	15,230,000	15,279,000
Less: Average participating securities (in shares)	(62,000)	(69,000)	(62,000)	(70,000)
Weighted average shares (in shares)	15,168,000	15,217,000	15,168,000	15,209,000
Basic earnings per common share (in dollar per share)	$ 0.64	$ 0.55	$ 1.26	$ 1.02
Diluted earnings per common share computation:
Net earnings allocated to common stock	$ 9,727	$ 8,402	$ 19,162	$ 15,465
Weighted average common shares outstanding for basic earnings per common share (in shares)	15,168,000	15,217,000	15,168,000	15,209,000
Add: Dilutive effects of assumed exercises of stock options (in shares)	0	0	0	0
Weighted average shares and dilutive potential common shares (in shares)	15,168,000	15,217,000	15,168,000	15,209,000
Diluted earnings per common share (in dollar per share)	$ 0.64	$ 0.55	$ 1.26	$ 1.02
Distributed earnings allocated to common stock
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 2,580	$ 2,510	$ 5,160	$ 5,019
Undistributed earnings allocated to common stock
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 7,147	$ 5,892	$ 14,002	$ 10,446