Earnings Per Share (Computation of Basic and Diluted) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income	$ 19,734	$ 25,532	$ 63,199	$ 73,460
Denominator:
Weighted-average shares outstanding - basic (shares)	18,403,000	18,181,000	18,360,000	18,126,000
Effect of dilutive shares (shares)	275,000	407,000	289,000	377,000
Weighted-average shares outstanding - diluted (shares)	18,678,000	18,588,000	18,649,000	18,503,000
Basic earnings per share (in dollars per share)	$ 1.07	$ 1.40	$ 3.44	$ 4.05
Diluted earnings per share (in dollars per share)	$ 1.06	$ 1.37	$ 3.39	$ 3.97
Anti-dilutive shares excluded (shares)		0