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Common Shareholders' Equity (Restricted Stock, Performance Share Awards and Options) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Restricted Stock and Performance Share Awards [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]      
Total fair value of awards vested $ 71 $ 53 $ 47
Restricted Stock Awards [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period (in years) 3 years    
Activity [Roll Forward]      
Nonvested balance at beginning of year (in shares) 1,032,288    
Granted (in shares) 498,587    
Vested (in shares) (544,094)    
Forfeited (in shares) (123,156)    
Nonvested balance at end of year (in shares) 863,625 1,032,288  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]      
Nonvested balance at beginning of year, weighted-average grant date fair value (in dollars per share) $ 50.84    
Granted, weighted-average grant date fair value (in dollars per share) $ 60.78 $ 54.77 $ 46.72
Vested, weighted-average grant date fair value (in dollars per share) $ 51.67    
Forfeited, weighted-average grant date fair value (in dollars per share) $ 55.54    
Nonvested balance at end of year, weighted-average grant date fair value (in dollars per share) $ 55.26 $ 50.84  
Performance Share Awards [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period (in years) 3 years    
Activity [Roll Forward]      
Nonvested balance at beginning of year (in shares) 1,351,074    
Granted (in shares) 720,669    
Vested (in shares) (616,130)    
Forfeited (in shares) (170,524)    
Nonvested balance at end of year (in shares) 1,285,089 1,351,074  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]      
Nonvested balance at beginning of year, weighted-average grant date fair value (in dollars per share) $ 43.72    
Granted, weighted-average grant date fair value (in dollars per share) $ 51.23 $ 50.13 $ 42.95
Vested, weighted-average grant date fair value (in dollars per share) $ 45.32    
Forfeited, weighted-average grant date fair value (in dollars per share) $ 47.58    
Nonvested balance at end of year, weighted-average grant date fair value (in dollars per share) $ 46.65 $ 43.72