Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic earnings per common share computation:
Net income per consolidated statements of income	$ 8,246	$ 9,767	$ 17,059	$ 19,240
Net earnings allocated to participating securities	(29)	(40)	(59)	(78)
Net earnings allocated to common stock	$ 8,217	$ 9,727	$ 17,000	$ 19,162
Weighted average common shares outstanding, including shares considered participating securities (in shares)	15,393,000	15,230,000	15,365,000	15,230,000
Less: Average participating securities (in shares)	(52,000)	(62,000)	(52,000)	(62,000)
Weighted average shares (in shares)	15,341,000	15,168,000	15,313,000	15,168,000
Basic earnings per common share (in dollar per share)	$ 0.54	$ 0.64	$ 1.11	$ 1.26
Diluted earnings per common share computation:
Net earnings allocated to common stock	$ 8,217	$ 9,727	$ 17,000	$ 19,162
Weighted average common shares outstanding for basic earnings per common share (in shares)	15,341,000	15,168,000	15,313,000	15,168,000
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)	15,341,000	15,168,000	15,313,000	15,168,000
Diluted earnings per common share (in dollar per share)	$ 0.54	$ 0.64	$ 1.11	$ 1.26
Distributed earnings allocated to common stock
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 2,608	$ 2,580	$ 5,216	$ 5,160
Undistributed earnings allocated to common stock
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 5,609	$ 7,147	$ 11,784	$ 14,002