Earning Per Common Share (Calculation of Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 12,962	$ 12,327	$ 37,943	$ 34,319
Less dividends and undistributed earnings allocated to participating securities	(28)	(25)	(84)	(72)
Net income applicable to common shareholders	$ 12,934	$ 12,302	$ 37,859	$ 34,247
Weighted average common shares outstanding - basic	17,212,000	17,090,000	17,201,000	17,060,000
Basic earnings per common share	$ 0.75	$ 0.72	$ 2.20	$ 2.01
Less dividends and undistributed earnings allocated to participating securities	$ (28)	$ (25)	$ (84)	$ (72)
Net income available to common shareholders	$ 12,934	$ 12,302	$ 37,859	$ 34,247
Dilutive effect of common stock equivalents	106,000	113,000	119,000	138,000
Weighted average common shares outstanding - diluted	17,318,000	17,203,000	17,320,000	17,198,000
Diluted earnings per common share	$ 0.75	$ 0.72	$ 2.19	$ 1.99
Antidilutive common stock equivalents	0	46,800	0	48,718