Earnings Per Share (Calculation of Basic and Diluted EPS) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income	$ 7,588	$ 13,236	$ 10,562	$ 8,659	$ 11,384	$ 14,756	$ 9,710	$ 7,359	$ 40,045	$ 43,209	$ 41,151
Average common shares:
Basic (weighted-average outstanding shares)									11,434,000	11,349,000	11,875,000
Dilutive potential common shares from stock options (in shares)									131,000	133,000	164,000
Diluted (weighted-average outstanding shares)	11,608,000	11,595,000	11,550,000	11,507,000	11,500,000	11,496,000	11,498,000	11,436,000	11,565,000	11,482,000	12,039,000
Basic earnings per share (in dollars per share)									$ 3.50	$ 3.81	$ 3.47
Diluted earnings per share (in dollars per share)	$ 0.65	$ 1.14	$ 0.92	$ 0.75	$ 0.99	$ 1.28	$ 0.84	$ 0.64	$ 3.46	$ 3.76	$ 3.42
Stock options excluded from diluted earnings per share calculation (in shares)									21,491	61,690	37,261