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Stock-Based Compensation (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended 1 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Jan. 31, 2012
Performance Share Units [Member]
Jan. 31, 2011
Performance Share Units [Member]
Dec. 31, 2012
Performance Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Maximum shares available for grants 4,000,000          
Share-based compensation expense $ 24 $ 14 $ 13      
Employee service share-based compensation, tax benefit from compensation expense 9 5 5      
Amounts paid to settle share units 11 4 2      
Compensation cost not yet recognized $ 21          
Expected weighted average recognition period for share-based compensation expense, in months 20 months          
Performance period           3 years
Percentage of shares issued per share unit, minimum           0.00%
Percentage of shares issued per share unit, maximum           200.00%
Fair value of each share unit, per share       $ 35.68 $ 31.41 $ 35.68 [1]
Closing common share price       $ 33.13 $ 28.19  
Three-year risk-free rate       0.41% 1.08%  
Volatility rate, minimum       17.00% 22.00%  
Volatility rate, maximum       31.00% 36.00%  
[1] Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in January 2012 under the 2006 Plan.