Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Notional Values For Derivatives Instrument (Details) - Designated as Hedging Instrument £ in Millions, zł in Millions, $ in Millions	Mar. 31, 2024 USD ($)	Mar. 31, 2024 PLN (zł)	Mar. 31, 2024 GBP (£)	Sep. 30, 2023 USD ($)	Sep. 30, 2023 PLN (zł)	Sep. 30, 2023 GBP (£)
Interest rate contracts
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Derivative, notional amount	$ 1,000.0			$ 2,000.0
Foreign currency forward contracts | Polish Zloty
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Derivative, notional amount	35.8	zł 156.1		34.0	zł 156.1
Foreign currency forward contracts | British Pound Sterling
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Derivative, notional amount	$ 147.6		£ 120.0	$ 206.9		£ 168.0