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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 | Dec. 31, 2020 | Sep. 30, 2020
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Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |
Derivative asset, fair value, gross asset | $ 3,170.4 | $ 3,264.4
Derivative liability, fair value, gross liability | 2,916.5 | 3,069.5
TBA Securities Sold | Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |
Derivative asset, fair value, gross asset | 25.6 | 10.8
Derivative asset, notional amount | 6,190.7 | 5,389.3
TBA Securities Sold | Payables to Broker-dealers, Clearing Organizations and Counterparties |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |
Derivative asset, fair value, gross asset | 0.0 | (1.7)
Derivative asset, notional amount | 136.9 | 2,647.7
Forward Settling Securities Purchased | Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |
Derivative liability, fair value, gross liability | 120.4 | 2,978.7
Derivative liability, fair value, gross liability | 0.0 | 2.8
Forward Settling Securities Purchased | Payables to Broker-dealers, Clearing Organizations and Counterparties |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |
Derivative liability, fair value, gross liability | 7,613.1 | 6,549.4
Derivative liability, fair value, gross liability | (30.8) | (13.0)
Forward Settling Securities Sold | Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |
Derivative asset, fair value, gross asset | 1.4 | 0.0
Derivative asset, notional amount | 1,392.7 | 0.0
Derivative liability, fair value, gross liability | 1,040.6 | 1,946.0
Derivative liability, fair value, gross liability | 1.2 | 17.4
Forward Settling Securities Purchased | Deposits and Receivables from Broker-dealers, Clearing Organizations and Counterparties |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |
Derivative asset, fair value, gross asset | 0.0 | (7.3)
Derivative asset, notional amount | $ 0.0 | $ 2,447.1

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