Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net Income Per Common Share
Number of outstanding options not included in computation of Diluted EPS (in shares)	0	0	0	6,000
Numerator
Net income	$ 50,477	$ 40,244	$ 85,263	$ 67,336
Denominator
Weighted average number of common shares outstanding	51,751,000	51,418,000	51,785,000	51,318,000
Effect of outstanding share-based awards	1,226,000	776,000	1,305,000	786,000
Weighted average number of shares for diluted net income per common share	52,977,000	52,194,000	53,090,000	52,104,000
Basic earnings per-share (in dollars per share)	$ 0.98	$ 0.78	$ 1.65	$ 1.31
Diluted earnings per-share (in dollars per share)	$ 0.95	$ 0.77	$ 1.61	$ 1.29