Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Dilutive shares (in shares)			0	0
Basic earnings per common share computation:
Net income per consolidated statements of income	$ 8,441	$ 6,683	$ 15,538	$ 13,163
Net earnings allocated to participating securities	(39)	(40)	(73)	(78)
Net earnings allocated to common stock	$ 8,402	$ 6,643	$ 15,465	$ 13,085
Weighted average common shares outstanding, including shares considered participating securities (in shares)	15,286,000	15,294,000	15,279,000	15,138,000
Less: Average participating securities (in shares)	(69,000)	(88,000)	(70,000)	(87,000)
Weighted average shares (in shares)	15,217,000	15,206,000	15,209,000	15,051,000
Basic earnings per common share (in dollars per share)	$ 0.55	$ 0.44	$ 1.02	$ 0.87
Diluted earnings per common share computation:
Net earnings allocated to common stock	$ 8,402	$ 6,643	$ 15,465	$ 13,085
Weighted average common shares outstanding for basic earnings per common share (in shares)	15,217,000	15,206,000	15,209,000	15,051,000
Diluted earnings per common share (in dollars per share)	$ 0.55	$ 0.44	$ 1.02	$ 0.87
Distributed earnings allocated to common stock
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 2,510	$ 2,507	$ 5,019	$ 5,015
Undistributed earnings allocated to common stock
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 5,892	$ 4,136	$ 10,446	$ 8,070