Earnings Per 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 Available to Common Shareholders	$ 9,669	$ 13,932	$ 14,685	$ 16,026	$ 14,656	$ 13,232	$ 12,541	$ 11,702	$ 54,310	$ 52,128	$ 54,097
Undistributed Earnings (Loss) Allocated to Participating Securities, Basic									591	623	613
Distributed earnings allocated to common shares outstanding									27,497	25,710	25,212
Undistributed earnings allocated to common shares outstanding									26,222	25,795	28,272
Net earnings allocated to common shareholders	$ 9,560	$ 13,778	$ 14,531	$ 15,849	$ 14,479	$ 13,075	$ 12,391	$ 11,571	$ 53,719	$ 51,505	$ 53,484
Average shares outstanding, basic	15,472,000	15,485,000	15,462,000	15,252,000	14,894,000	14,899,000	14,889,000	14,916,000	15,412,000	14,900,000	15,123,000
Effect of dilutive securities									24,000	13,000	48,000
Average shares outstanding, diluted	15,497,000	15,505,000	15,487,000	15,277,000	14,914,000	14,909,000	14,902,000	14,927,000	15,436,000	14,913,000	15,171,000
Basic earnings per share	$ 0.62	$ 0.89	$ 0.94	$ 1.04	$ 0.97	$ 0.88	$ 0.83	$ 0.78	$ 3.49	$ 3.46	$ 3.54
Diluted earnings per share	$ 0.62	$ 0.89	$ 0.94	$ 1.04	$ 0.97	$ 0.88	$ 0.83	$ 0.78	$ 3.48	$ 3.45	$ 3.53
Options to purchase shares of common stock, anti-dilutive									3,000	2,000	13,000