Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Fair Values of Derivative Instruments Designated for Hedging Held (Details) - USD ($) $ in Millions	Mar. 31, 2022	Sep. 30, 2021
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 12,036.2	$ 5,760.7
Liability Derivatives	11,843.3	5,628.1
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.0	0.0
Liability Derivatives	24.3	$ 0.0
Designated as Hedging Instrument | Interest rate contracts
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.0
Designated as Hedging Instrument | Interest rate contracts | Financial instruments sold, not yet purchased
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ 24.3