Net Income Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net Income Per Common Share
Number of outstanding options not included in computation of Diluted EPS (in shares)	3,164,000	3,945,000	3,300,000	3,963,000
Numerator
Net Income	$ 26,394	$ 20,933	$ 50,348	$ 37,211
Denominator
Weighted average number of common shares outstanding	51,881,000	51,241,000	51,841,000	51,022,000
Effect of outstanding share-based awards	729,000	912,000	710,000	586,000
Weighted average number of shares for diluted net income per common share	52,610,000	52,153,000	52,551,000	51,608,000
Basic earnings per share (in dollars per share)	$ 0.51	$ 0.41	$ 0.97	$ 0.73
Diluted earnings per share (in dollars per share)	$ 0.50	$ 0.40	$ 0.96	$ 0.72