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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
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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

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