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Common Shareholders' Equity (Restricted Stock, Performance Share Awards and Options) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
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) 579,497    
Granted (in shares) 303,150    
Vested (in shares) (274,620)    
Forfeited (in shares) (44,367)    
Nonvested balance at end of year (in shares) 563,660 579,497  
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) $ 75.65    
Granted, weighted-average grant date fair value (in dollars per share) 103.58 $ 93.46 $ 74.02
Vested, weighted-average grant date fair value (in dollars per share) 73.92    
Forfeited, weighted-average grant date fair value (in dollars per share) 99.99    
Nonvested balance at end of year, weighted-average grant date fair value (in dollars per share) $ 89.60 $ 75.65  
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) 996,227    
Granted (in shares) 567,437    
Vested (in shares) (609,321)    
Forfeited (in shares) (39,144)    
Nonvested balance at end of year (in shares) 915,199 996,227  
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) $ 67.19    
Granted, weighted-average grant date fair value (in dollars per share) 77.12 $ 71.52 $ 58.53
Vested, weighted-average grant date fair value (in dollars per share) 53.55    
Forfeited, weighted-average grant date fair value (in dollars per share) 79.36    
Nonvested balance at end of year, weighted-average grant date fair value (in dollars per share) $ 81.90 $ 67.19  
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 $ 108 $ 85 $ 82