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Investments - Summary of Investments (Details) - USD ($)
$ in Millions
Sep. 30, 2025
Sep. 30, 2024
Investment Holdings [Line Items]    
Total investments, at fair value $ 1,179.5 $ 838.0
Investments in equity method investees 893.9 1,219.7
Other investments 300.6 280.7
Total 2,374.0 2,338.4
Sponsored funds and separate accounts [Member]    
Investment Holdings [Line Items]    
Investments, at fair value 809.6 509.1
Investment related to long-term incentive plans [Member]    
Investment Holdings [Line Items]    
Investments, at fair value 288.1 271.6
Other equity and debt investments [Member]    
Investment Holdings [Line Items]    
Investments, at fair value $ 81.8 $ 57.3