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Share-Based Payments - Incentive Plans (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Stock Incentive Plans      
Employer contributions, compensation expense $ 33.7 $ 32.3 $ 32.3
Employee Stock Option      
Stock Incentive Plans      
Securities available for future issuance (in shares) 12.0    
Expiration term of unexercised options 7 years    
Number of Options      
Balance at the beginning of the period (in shares) 0.1 0.6 0.7
Granted (in shares) 0.3    
Exercised (in shares)     (0.1)
Cancelled (in shares)   (0.5)  
Balance at the end of the period (in shares) 0.4 0.1 0.6
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 $ 31.62 $ 31.11
Granted (in dollars per share) 38.72    
Exercised (in dollars per share)     27.79
Cancelled (in dollars per share)   (31.62)  
Balance at the end of the period (in dollars per share) 36.41 31.62 $ 31.62
Exercisable at the end of the period (in dollars per share) $ 31.62 $ 31.62