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Share-Based Payments - Incentive Plans (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Stock Incentive Plans        
Employer contributions, compensation expense   $ 32.3 $ 32.3 $ 32.9
Employee Stock Option        
Stock Incentive Plans        
Securities available for future issuance (in shares)   11.6    
Expiration term of unexercised options   7 years    
Number of Options        
Balance at the beginning of the period (in shares)   0.6 0.7 0.9
Exercised (in shares)     (0.1) (0.2)
Cancelled (in shares) (0.5)      
Balance at the end of the period (in shares)   0.1 0.6 0.7
Exercisable at the end of the period (in shares)   0.1   0.1
Stock options, Weighted Average Exercise Price        
Balance at the beginning of the period (in dollars per share)   $ 31.62 $ 31.11 $ 30.36
Exercised (in dollars per share)     27.79 26.42
Cancelled (in dollars per share) $ (31.62)      
Balance at the end of the period (in dollars per share)   31.62 $ 31.62 31.11
Exercisable at the end of the period (in dollars per share)   $ 31.62   $ 27.79