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Stockholders' Equity (Tables)
 12 Months Ended
Dec. 31, 2012
Stockholders' Equity (Tables) [Abstract]  
Summary of common stock activity
The following table summarizes common stock activity from January 1, 2010 to December 31, 2012:
 
2012
 
2011
 
2010
Shares outstanding at the beginning of the year
271.1

 
270.2

 
268.2

Stock options exercised
0.2

 
0.3

 
1.5

Stock grants to employees
1.5

 
0.7

 
0.6

Employee stock purchases
0.3

 
0.2

 
0.2

Shares relinquished
(0.3
)
 
(0.3
)
 
(0.3
)
Shares repurchased
(4.2
)
 
—

 
—

Shares outstanding at the end of the year
268.6

 
271.1

 
270.2