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Share-Based Payments - Restricted stock unit, PEP unit, and Stock Option activity (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 6 Months Ended 12 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Sep. 30, 2023
Other disclosures      
Unrecognized compensation expense $ 78.7   $ 48.3
Recognition period of unrecognized compensation expense 3 years   3 years
Restricted Stock Units      
Units      
Outstanding at beginning (in units) 0.8 1.0 1.0
Granted (in units) 0.3 0.3  
Vested (in shares) (0.3) (0.4)  
Outstanding at ending (in units) 0.8 0.9 0.8
Weighted Average Grant-Date Fair Value      
Outstanding at beginning (in USD per unit) $ 68.34 $ 53.05 $ 53.05
Granted (in USD per unit) 92.32 83.64  
Vested (in USD per unit) 49.99 44.95  
Outstanding at ending (in USD per unit) $ 83.90 $ 65.75 $ 68.34
Other disclosures      
Compensation expense related to share-based payments $ 30.6 $ 24.6  
PEP Units      
Share-based Payments      
Vesting period 3 years    
Units      
Outstanding at beginning (in units) 0.7 0.7 0.7
Granted (in units) 0.2 0.2  
PEP units earned (in units) 0.2 0.2  
Vested (in shares) (0.4) (0.4)  
Outstanding at ending (in units) 0.7 0.7 0.7
Weighted Average Grant-Date Fair Value      
Outstanding at beginning (in USD per unit) $ 75.54 $ 60.60 $ 60.60
Granted (in USD per unit) 104.82 94.65  
PEP units earned (in USD per unit) 52.49 43.19  
Vested (in USD per unit) 52.49 43.19  
Outstanding at ending (in USD per unit) $ 95.37 $ 75.53 $ 75.54