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Investments - Summary of Investments (Details) - USD ($)
$ in Millions
Mar. 31, 2025
Sep. 30, 2024
Investment Holdings [Line Items]    
Total investments, at fair value $ 636.4 $ 838.0
Investments in equity method investees 1,178.7 1,219.7
Other investments 414.3 280.7
Investments 2,229.4 2,338.4
Sponsored funds and separate accounts [Member]    
Investment Holdings [Line Items]    
Investments, at fair value 278.0 509.1
Investments related to long-term incentive plans [Member]    
Investment Holdings [Line Items]    
Investments, at fair value 258.2 271.6
Other equity and debt investments [Member]    
Investment Holdings [Line Items]    
Investments, at fair value $ 100.2 $ 57.3