Earnings Per Share (Computation of Basic and Diluted) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income	$ 25,532	$ 16,065	$ 73,460	$ 36,370
Denominator:
Weighted-average shares outstanding - basic (shares)	18,181,000	17,996,000	18,126,000	17,990,000
Effect of dilutive shares (shares)	407,000	187,000	377,000	227,000
Weighted-average shares outstanding - diluted (shares)	18,588,000	18,183,000	18,503,000	18,217,000
Basic earnings per share (in dollars per share)	$ 1.40	$ 0.89	$ 4.05	$ 2.02
Diluted earnings per share (in dollars per share)	$ 1.37	$ 0.88	$ 3.97	$ 2.00
Anti-dilutive shares excluded (shares)	0	13,500