Earning Per Common Share (Calculation of Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net income	$ 17,273	$ 17,674	$ 34,768	$ 33,885
Less dividends and undistributed earnings allocated to participating securities	(35)	(38)	(69)	(76)
Net income applicable to common shareholders	$ 17,238	$ 17,636	$ 34,699	$ 33,809
Weighted average common shares outstanding - basic	17,330,000	17,272,000	17,317,000	17,253,000
Basic earnings per common share	$ 0.99	$ 1.02	$ 2.00	$ 1.96
Less dividends and undistributed earnings allocated to participating securities	$ (35)	$ (38)	$ (69)	$ (76)
Net income applicable to common shareholders	$ 17,238	$ 17,636	$ 34,699	$ 33,809
Dilutive effect of common stock equivalents	75,000	115,000	86,000	131,000
Weighted average common shares outstanding - diluted	17,405,000	17,387,000	17,403,000	17,384,000
Diluted earnings per common share	$ 0.99	$ 1.01	$ 1.99	$ 1.94
Antidilutive common stock equivalents	87,950	43,871	87,863	48,956