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Common Equity - Stock Options (Details) - Stock options - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
Jan. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Options Activity        
Outstanding, shares, beginning balance 4,644,214 5,122,775    
Granted, shares   552,215 794,764 516,475
Exercised, shares   (1,019,111)    
Forfeited, shares   (11,665)    
Outstanding, shares, ending balance   4,644,214 5,122,775  
Options - Weighted Average Exercise Price        
Outstanding, Weighted-Average Exercise Price, Beginning $ 43.11 $ 38.95    
Granted, Weighted-Average Exercise Price   58.31    
Exercised, Weighted-Average Exercise Price   30.24    
Forfeited, Weighted-Average Exercise Price   56.48    
Outstanding, Weighted-Average Exercise Price, Ending   $ 43.11 $ 38.95  
Options - Additional Disclosures        
Outstanding, Weighted-Average Remaining Contractual Life (Years)   6 years    
Outstanding, Aggregate Intrinsic Value   $ 108.3    
Exercisable, shares   3,275,850    
Exercisable, Weighted-Average Exercise Price (in dollars per share)   $ 38.23    
Exercisable, Weighted-Average Remaining Contractual Life (Years)   5 years    
Exercisable, Aggregate Intrinsic Value   $ 92.4    
Intrinsic value of options exercised   33.8 $ 55.4 $ 36.1
Tax benefit from option exercises   13.5 $ 22.2 $ 14.5
Compensation cost not yet recognized   $ 2.7    
Weighted-average period over which unrecognized compensation cost is expected to be recognized   1 year 8 months    
Estimated weighted-average fair value per stock option (in dollars per share)   $ 7.45 $ 5.14 $ 5.29
Subsequent event        
Options Activity        
Granted, shares 660,655      
Options - Weighted Average Exercise Price        
Granted, Weighted-Average Exercise Price $ 66.02      
Options - Additional Disclosures        
Estimated weighted-average fair value per stock option (in dollars per share) $ 7.84