Earnings Per Share (Computation of Basic and Diluted) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income	$ 17,329	$ 20,896	$ 43,465	$ 47,928
Denominator:
Weighted-average shares outstanding - basic (shares)	18,389,000	18,140,000	18,338,000	18,098,000
Effect of dilutive shares (shares)	271,000	407,000	297,000	362,000
Weighted-average shares outstanding - diluted (shares)	18,660,000	18,547,000	18,635,000	18,460,000
Basic earnings per share (in dollars per share)	$ 0.94	$ 1.15	$ 2.37	$ 2.65
Diluted earnings per share (in dollars per share)	$ 0.93	$ 1.13	$ 2.33	$ 2.60
Anti-dilutive shares excluded (shares)	27,145	0