Net Income Per Common Share (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
Net Income Per Common Share
Number of outstanding options not included in computation of Diluted EPS (in shares)				207,000	284,000
Numerator
Net income	$ 83,046	$ 53,716	[1]	$ 213,266	$ 138,978
Denominator
Weighted average number of common shares outstanding	52,039,000	51,833,000	[1]	52,002,000	51,801,000
Effect of outstanding share-based awards	942,000	1,247,000		974,000	1,286,000
Weighted average number of shares for diluted net income per common share	52,981,000	53,080,000	[1]	52,976,000	53,087,000
Basic earnings per-share (in dollars per share)	$ 1.60	$ 1.04	[1]	$ 4.10	$ 2.68
Diluted earnings per-share (in dollars per share)	$ 1.57	$ 1.01	[1]	$ 4.03	$ 2.62

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.