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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2012
Share-based Compensation [Abstract]  
Net stock-based compensation expense
Net stock-based compensation expense recognized was as follows (in millions):
 
 
For the Three Months Ended
September 30,
 
For the Nine Months Ended
September 30,
 
2012
 
2011
 
2012
 
2011
Net stock-based compensation expense
$
2.1

 
$
3.5

 
$
10.4

 
$
12.6