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Stock Based Compensation (Tables)
6 Months Ended
Jun. 30, 2011
Stock Based Compensation (Tables) [Abstract]  
Stock Based Compensation Expense
                                 
    For the Three Months     For the Six Months  
    Ended June 30,     Ended June 30,  
    2011     2010     2011     2010  
Net stock-based compensation expense
  $ 4.2     $ 3.4     $ 9.1     $ 7.7