Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 18,566	$ 18,322	$ 21,040	$ 11,901	$ 15,536	$ 18,814	$ 17,273	$ 17,495	$ 69,829	$ 69,118	$ 68,432
Less: dividends and undistributed earnings allocated to participating securities									(152)	(139)	(144)
Net income available to common shareholders									$ 69,677	$ 68,979	$ 68,288
Weighted average common shares outstanding - basic	17,264,000	17,260,000	17,257,000	17,345,000	17,351,000	17,338,000	17,330,000	17,304,000	17,282,000	17,331,000	17,272,000
Basic earnings per common share (in dollars per share)	$ 1.07	$ 1.06	$ 1.22	$ 0.68	$ 0.89	$ 1.08	$ 0.99	$ 1.01	$ 4.03	$ 3.98	$ 3.95
Less dividends and undistributed earnings allocated to participating securities									$ (151)	$ (139)	$ (144)
Net income available to common shareholders	$ 18,524	$ 18,285	$ 21,000	$ 11,869	$ 15,502	$ 18,778	$ 17,238	$ 17,461	$ 69,678	$ 68,979	$ 68,288
Dilutive effect of common stock equivalents (in shares)									120,000	83,000	119,000
Weighted average common shares outstanding - diluted	17,360,000	17,317,000	17,292,000	17,441,000	17,436,000	17,414,000	17,405,000	17,401,000	17,402,000	17,414,000	17,391,000
Diluted earnings per common share (in dollars per share)	$ 1.07	$ 1.06	$ 1.21	$ 0.68	$ 0.89	$ 1.08	$ 0.99	$ 1.00	$ 4.00	$ 3.96	$ 3.93
Antidilutive common stock equivalents									223,506	100,643	55,821