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Income Taxes (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Income Taxes    
Income tax provision$ 10,649$ 7,515$ 32,113$ 22,961
Effective income tax rate (as a percent)16.50%17.10%18.00%17.60%