Securities and Commodity Financing Transactions - Fair Values of Collateral Pledged, Received and Repledged (Details) - USD ($) $ in Millions |
Mar. 31, 2025 |
Sep. 30, 2024 |
|---|---|---|
| Assets Sold under Agreements to Repurchase [Line Items] | ||
| Securities pledged or repledged to cover collateral requirements for tri-party arrangements | $ 8,200.9 | $ 6,767.1 |
| Securities received as collateral that may be repledged | 25,714.7 | 20,126.8 |
| Securities received as collateral that may be repledged covering securities sold short | 2,966.4 | 2,408.3 |
| Asset Pledged as Collateral with Right | ||
| Assets Sold under Agreements to Repurchase [Line Items] | ||
| Repledged securities borrowed and client securities held under custodial clearing arrangements to collateralize securities loaned agreements | 1,577.3 | 1,533.3 |
| Tri-party Repurchase Agreements | ||
| Assets Sold under Agreements to Repurchase [Line Items] | ||
| Securities pledged or repledged to cover collateral requirements for tri-party arrangements | $ 8,385.3 | $ 6,777.9 |