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
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)

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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