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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - TBAs and Forward Settling Securities (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Sep. 30, 2019
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Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | $ 3,264.4 | $ 3,671.5
Derivative liability, fair value, gross liability | (3,069.5) | (3,604.5)
TBA securities purchased [Member] | Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 10.8 | 3.7
Derivative asset, notional amount | (5,389.3) | (1,778.4)
Derivative liability, fair value, gross liability | (1.7) | (0.6)
Derivative liability, notional amount | (2,647.7) | (234.5)
TBA securities sold [Member] | Payables to Broker-dealers, Clearing Organizations and Counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 2.8 | 0.9
Derivative asset, notional amount | (2,978.7) | (451.6)
Derivative liability, fair value, gross liability | (13.0) | (5.9)
Derivative liability, notional amount | (6,549.4) | (2,788.0)
Forward settling securities purchased [Member] | Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative liability, fair value, gross liability | (7.3) | 0.0
Derivative liability, notional amount | (2,447.1) | 0.0
Forward settling securities purchased [Member] | Payables to Broker-dealers, Clearing Organizations and Counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative liability, fair value, gross liability | 0.0 | (0.3)
Derivative liability, notional amount | 0.0 | (1,243.5)
Forward settling securities sold [Member] | Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Derivative asset, fair value, gross asset | 17.4 | 5.2
Derivative asset, notional amount | $ (1,946.0) | $ (581.2)

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