Common Stock and Earnings per Share (Schedule of Calculations of Basic and Diluted 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
Basic
Net income	$ 5,331	$ 6,863	$ 6,513	$ 6,307	$ 6,424	$ 6,222	$ 5,869	$ 5,833	$ 25,014	$ 24,348	$ 23,056
Weighted average common shares outstanding (in shares)									12,250,465	12,265,434	12,494,466
Basic earnings per share (in dollars per share)	$ 0.44	$ 0.56	$ 0.53	$ 0.51	$ .53	$ .51	$ .48	$ .47	$ 2.04	$ 1.99	$ 1.85
Diluted
Net income	$ 5,331	$ 6,863	$ 6,513	$ 6,307	$ 6,424	$ 6,222	$ 5,869	$ 5,833	$ 25,014	$ 24,348	$ 23,056
Weighted average common shares outstanding (in shares)									12,250,465	12,265,434	12,494,466
Effect of dilutive restricted stock, PBRS and SARs (in shares)									179,445	172,386	175,784
Weighted average common shares outstanding assuming dilution (in shares)									12,429,910	12,437,820	12,670,250
Diluted earnings per share (in dollars per share)	$ 0.43	$ 0.55	$ 0.52	$ 0.51	$ .52	$ .50	$ .47	$ .47	$ 2.01	$ 1.96	$ 1.82