Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Accumulated Other Comprehensive Income Effects of Derivative Instruments (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Mar. 31, 2025 |
Mar. 31, 2024 |
Mar. 31, 2025 |
Mar. 31, 2024 |
|
| Derivative Instruments and Hedging Activities Disclosure [Line Items] | ||||
| Amount of Gain/(Loss) Recognized in Other Comprehensive Income on Derivatives, net of tax | $ 6.6 | $ (0.8) | $ (0.3) | $ 19.9 |
| Interest rate contracts | ||||
| Derivative Instruments and Hedging Activities Disclosure [Line Items] | ||||
| Amount of Gain/(Loss) Recognized in Other Comprehensive Income on Derivatives, net of tax | 0.0 | 1.7 | 0.1 | 14.3 |
| Foreign currency forward contracts | ||||
| Derivative Instruments and Hedging Activities Disclosure [Line Items] | ||||
| Amount of Gain/(Loss) Recognized in Other Comprehensive Income on Derivatives, net of tax | $ 6.6 | $ (2.5) | $ (0.4) | $ 5.6 |