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Stock-Based Compensation Plans - Stock Option award activity (Details) - Post Stock Options - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2022
Share-based Payment Award [Line Items]      
Outstanding 351,081 748,708  
Granted 0    
Exercised (397,627)    
Forfeited 0    
Expired 0    
Vested and Expected to Vest 351,081    
Exercisable 351,081    
Outstanding, Weighted Average Exercise Price $ 57.99 $ 51.96  
Granted, Weighted Average Exercise Price 0    
Exercised, Weighted Average Exercise Price 46.64    
Forfeited, Weighted Average Exercise Price 0    
Expired, Weighted Average Exercise Price 0    
Vested and Expected to Vest, Weighted Average Exercise Price 57.99    
Exercisable, Weighted Average Exercise Price $ 57.99    
Outstanding, Weighted Average Remaining Contractual Term 3 years 7 months 9 days    
Vested and Expected to Vest, Weighted Average Remaining Contractual Term 3 years 7 months 9 days    
Exercisable, Weighted Average Remaining Contractual Term 3 years 7 months 9 days    
Outstanding, Intrinsic Value $ 20.3    
Vested and Expected to Vest, Aggregate Intrinsic Value 20.3    
Exercisable, Intrinsic Value 20.3    
Exercised, Total Intrinsic Value 24.2 $ 14.1 $ 14.1
Proceeds from Stock Options Exercised $ 2.9 $ 3.9 $ 4.9