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Income Taxes (Details) (USD $)
3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Income Taxes    
Income tax provision $ 6,615,000 $ 10,415,000
Effective income tax rate (as a percent) 12.00% 20.00%
Income tax benefit due to change in estimate related to uncertain tax position $ 3,300,000