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
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

Nominal or face amount used to calculate payments on the derivative asset.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
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