Earnings Per Share (Calculation of Basic and Diluted EPS) (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	$ 3,240	$ 16,592	$ 12,316	$ 12,167	$ 7,588	$ 13,236	$ 10,562	$ 8,659	$ 44,315	$ 40,045	$ 43,209
Average common shares:
Basic (weighted-average outstanding shares)									11,549,000	11,434,000	11,349,000
Dilutive potential common shares from stock options (in shares)									133,000	131,000	133,000
Diluted (weighted-average outstanding shares)	11,727,000	11,708,000	11,671,000	11,620,000	11,608,000	11,595,000	11,550,000	11,507,000	11,682,000	11,565,000	11,482,000
Basic earnings per share (in dollars per share)									$ 3.84	$ 3.50	$ 3.81
Diluted earnings per share (in dollars per share)	$ 0.27	$ 1.42	$ 1.05	$ 1.05	$ 0.65	$ 1.14	$ 0.92	$ 0.75	$ 3.79	$ 3.46	$ 3.76
Stock options excluded from diluted earnings per share calculation (in shares)									1,565	21,491	61,690