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Earnings Per Share (Tables)
9 Months Ended
Sep. 30, 2011
Earnings Per Share (Tables) [Abstract] 
Common share data used for computing the basic and diluted earnings per share
 
Basic and diluted earnings per share were computed using the following common share data (shares in millions):
 
                                 
    Three Months
    Nine Months
 
    Ended September 30,     Ended September 30,  
    2011     2010     2011     2010  
 
Number of common shares outstanding at end of period
    461.2       475.7       461.2       475.7  
Effect of using weighted average common shares outstanding
    7.1       1.6       11.5       6.0  
                                 
Weighted average basic common shares outstanding
    468.3       477.3       472.7       481.7  
Dilutive effect of equity-based compensation awards and other contingently issuable shares
    1.4       3.7       1.8       3.2  
                                 
Weighted average diluted common shares outstanding
    469.7       481.0       474.5       484.9  
                                 
Potentially issuable shares
    17.3       14.7       17.3       14.7  
Number of anti-dilutive potentially issuable shares excluded from diluted common shares outstanding
    9.9       0.2       6.4       0.2