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Investments - Narratives (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Investments [Abstract]      
Securities Sold under Agreements to Repurchase $ 200.5 $ 111.4  
Securities Borrowed, Fair Value of Collateral 206.4 121.7  
Net gains (losses) recognized on equity securities measured at fair value and trading debt securities $ 5.4 $ 108.1 $ 66.1