Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 17,036	$ 17,511	$ 17,674	$ 16,211	$ 7,982	$ 12,962	$ 13,199	$ 11,782	$ 68,432	$ 45,925	$ 46,481
Less dividends and undistributed earnings allocated to participating securities									(144)	(108)	(97)
Net income available to common shareholders									$ 68,288	$ 45,817	$ 46,384
Weighted average common shares outstanding - basic	17,297,000	17,283,000	17,272,000	17,234,000	17,223,000	17,212,000	17,206,000	17,186,000	17,272,000	17,207,000	17,081,000
Basic earnings per common share (in dollars per share)	$ 0.98	$ 1.01	$ 1.02	$ 0.94	$ 0.46	$ 0.75	$ 0.77	$ 0.68	$ 3.95	$ 2.66	$ 2.72
Less dividends and undistributed earnings allocated to participating securities									$ (144)	$ (108)	$ (97)
Net income available to common shareholders	$ 17,004	$ 17,475	$ 17,636	$ 16,173	$ 7,958	$ 12,934	$ 13,170	$ 11,755	$ 68,288	$ 45,817	$ 46,384
Dilutive effect of common stock equivalents (in shares)									119,000	131,000	127,000
Weighted average common shares outstanding - diluted (in shares)	17,385,000	17,382,000	17,387,000	17,345,000	17,349,000	17,318,000	17,316,000	17,293,000	17,391,000	17,338,000	17,208,000
Diluted earnings per common share (in dollars per share)	$ 0.98	$ 1.01	$ 1.01	$ 0.93	$ 0.46	$ 0.75	$ 0.76	$ 0.68	$ 3.93	$ 2.64	$ 2.70
Antidilutive common stock equivalents									55,821	11,572	59,158