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Goodwill and Other Intangible Assets - Narrative (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Goodwill and Intangible Assets Disclosure [Abstract]      
Definite-lived Intangible Assets, Weighted Average Useful Life 8 years 7 months 6 days    
Impairment of intangible asset $ 226.6 $ 389.2 $ 0.0
Finite-Lived Intangible assets, Reclass from Indefinite-lived 223.9    
Indefinite-lived intangible assets 3,400.3 3,851.5  
Indefinite-Lived Intangible Assets [Line Items]      
Impairment of intangible asset 226.6 389.2 $ 0.0
Indefinite-lived intangible assets 3,400.3 $ 3,851.5  
Finite-Lived Intangible Assets [Line Items]      
Finite-Lived Intangible assets, Reclass from Indefinite-lived 223.9    
WAM      
Goodwill and Intangible Assets Disclosure [Abstract]      
Impairment of intangible asset 200.0    
Finite-Lived Intangible assets, Reclass from Indefinite-lived 125.0    
Indefinite-lived intangible assets 450.0    
Indefinite-Lived Intangible Assets [Line Items]      
Impairment of intangible asset 200.0    
Indefinite-lived intangible assets 450.0    
Finite-Lived Intangible Assets [Line Items]      
Finite-Lived Intangible assets, Reclass from Indefinite-lived 125.0    
BGIM & MC      
Goodwill and Intangible Assets Disclosure [Abstract]      
Impairment of intangible asset 24.4    
Indefinite-Lived Intangible Assets [Line Items]      
Impairment of intangible asset $ 24.4