Earning Per Common Share (Calculation of Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 17,674	$ 13,199	$ 33,885	$ 24,981
Less dividends and undistributed earnings allocated to participating securities	(38)	(29)	(76)	(56)
Net income applicable to common shareholders	$ 17,636	$ 13,170	$ 33,809	$ 24,925
Weighted average common shares outstanding - basic	17,272,000	17,206,000	17,253,000	17,196,000
Basic earnings per common share	$ 1.02	$ 0.77	$ 1.96	$ 1.45
Less dividends and undistributed earnings allocated to participating securities	$ (38)	$ (29)	$ (76)	$ (56)
Net income applicable to common shareholders	$ 17,636	$ 13,170	$ 33,809	$ 24,925
Dilutive effect of common stock equivalents	115,000	110,000	131,000	116,000
Weighted average common shares outstanding - diluted	17,387,000	17,316,000	17,384,000	17,312,000
Diluted earnings per common share	$ 1.01	$ 0.76	$ 1.94	$ 1.44
Antidilutive common stock equivalents	43,871	3,475	48,956	0