Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic earnings per common share computation:
Net income per consolidated statements of income	$ 10,357	$ 9,236	$ 29,597	$ 24,774
Net earnings allocated to participating securities	(37)	(40)	(114)	(113)
Net earnings allocated to common stock	$ 10,320	$ 9,196	$ 29,483	$ 24,661
Weighted average common shares outstanding, including shares considered participating securities (in shares)	15,197,000	15,285,000	15,218,000	15,281,000
Less: Average participating securities (in shares)	(51,000)	(60,000)	(58,000)	(67,000)
Weighted average shares (in shares)	15,146,000	15,225,000	15,160,000	15,214,000
Basic earnings per common share (in dollar per share)	$ 0.68	$ 0.60	$ 1.94	$ 1.62
Diluted earnings per common share computation:
Net earnings allocated to common stock	$ 10,320	$ 9,196	$ 29,483	$ 24,661
Weighted average common shares outstanding for basic earnings per common share (in shares)	15,146,000	15,225,000	15,160,000	15,214,000
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)	15,146,000	15,225,000	15,160,000	15,214,000
Diluted earnings per common share (in dollar per share)	$ 0.68	$ 0.60	$ 1.94	$ 1.62
Distributed earnings allocated to common stock
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 2,573	$ 2,586	$ 7,734	$ 7,607
Undistributed earnings allocated to common stock
Basic earnings per common share computation:
Net earnings allocated to common stock	$ 7,747	$ 6,610	$ 21,749	$ 17,054