Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 7,982	$ 12,962	$ 13,199	$ 11,782	$ 12,162	$ 12,327	$ 11,057	$ 10,935	$ 45,925	$ 46,481	$ 43,465
Less dividends and undistributed earnings allocated to participating securities									(108)	(97)	(126)
Net income applicable to common shareholders									$ 45,817	$ 46,384	$ 43,339
Weighted average common shares outstanding - basic	17,223,000	17,212,000	17,206,000	17,186,000	17,142,000	17,090,000	17,067,000	17,023,000	17,207,000	17,081,000	16,879,000
Basic earnings per common share (in dollars per share)	$ 0.46	$ 0.75	$ 0.77	$ 0.68	$ 0.71	$ 0.72	$ 0.65	$ 0.64	$ 2.66	$ 2.72	$ 2.57
Less dividends and undistributed earnings allocated to participating securities									$ (108)	$ (97)	$ (126)
Net income applicable to common shareholders									$ 45,817	$ 46,384	$ 43,339
Dilutive effect of common stock equivalents (in shares)									131,000	127,000	188,000
Weighted average common shares outstanding - diluted (in shares)	17,349,000	17,318,000	17,316,000	17,293,000	17,245,000	17,203,000	17,194,000	17,157,000	17,338,000	17,208,000	17,067,000
Diluted earnings per common share (in dollars per share)	$ 0.46	$ 0.75	$ 0.76	$ 0.68	$ 0.70	$ 0.72	$ 0.64	$ 0.64	$ 2.64	$ 2.70	$ 2.54
Antidilutive common stock equivalents									11,572	59,158	34,850