Earning Per Common Share (Calculation of Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net income	$ 18,322	$ 18,814	$ 51,263	$ 53,582
Less dividends and undistributed earnings allocated to participating securities	(37)	(36)	(109)	(105)
Net income available to common shareholders	$ 18,285	$ 18,778	$ 51,154	$ 53,477
Weighted average common shares outstanding - basic	17,260,000	17,338,000	17,287,000	17,324,000
Basic earnings per common share	$ 1.06	$ 1.08	$ 2.96	$ 3.09
Less dividends and undistributed earnings allocated to participating securities	$ (37)	$ (36)	$ (109)	$ (105)
Net income available to common shareholders	$ 18,285	$ 18,778	$ 51,154	$ 53,477
Dilutive effect of common stock equivalents	57,000	76,000	82,000	82,000
Weighted average common shares outstanding - diluted	17,317,000	17,414,000	17,369,000	17,406,000
Diluted earnings per common share	$ 1.06	$ 1.08	$ 2.95	$ 3.07
Antidilutive common stock equivalents	223,210	93,075	221,047	87,775