Net Income Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Net Income Per Common Share
Number of outstanding options not included in computation of Diluted EPS (in shares)	3,298,000	3,952,000	3,368,000	3,971,000
Numerator
Net Income	$ 20,720	$ 16,960	$ 23,955	$ 16,278
Denominator
Weighted average number of common shares outstanding	51,881,000	50,944,000	51,822,000	50,912,000
Effect of outstanding share-based awards	666,000	845,000	700,000	423,000
Weighted average number of shares for diluted net income per common share	52,547,000	51,789,000	52,522,000	51,335,000
Basic earnings per share (in dollars per share)	$ 0.40	$ 0.33	$ 0.46	$ 0.32
Diluted earnings per share (in dollars per share)	$ 0.39	$ 0.33	$ 0.46	$ 0.32