Net Income Per Common Share (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
Net Income Per Common Share
Number of outstanding options not included in computation of Diluted EPS (in shares)				2,000
Numerator
Net income	$ 53,716	$ 41,322	$ 138,978	$ 108,659
Denominator
Weighted average number of common shares outstanding	51,833,000	51,627,000	51,801,000	51,421,000
Effect of outstanding share-based awards	1,247,000	844,000	1,286,000	803,000
Weighted average number of shares for diluted net income per common share	53,080,000	52,471,000	53,087,000	52,224,000
Basic earnings per-share (in dollars per share)	$ 1.04	$ 0.80	$ 2.68	$ 2.11
Diluted earnings per-share (in dollars per share)	$ 1.01	$ 0.79	$ 2.62	$ 2.08