Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - TBAs and Forward Settling Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Derivative Asset, Fair Value, Gross Asset	$ 7,350.3	$ 3,671.5
Derivative Liability, Fair Value, Gross Liability	(7,707.9)	(3,604.5)
TBA securities purchased [Member]
Derivative Liability, Notional Amount	(554.9)	(234.5)
Derivative Asset, Notional Amount	(3,335.4)	(1,778.4)
TBA securities sold [Member]
Derivative Liability, Notional Amount	(4,708.7)	(2,788.0)
Derivative Asset, Notional Amount	(559.9)	(451.6)
Forward settling securities purchased [Member]
Derivative Asset, Notional Amount	(896.9)	0.0
Over the Counter (OTC) Commodity Contracts [Member]
Derivative Liability, Notional Amount	0.0	(1,243.5)
Derivative Asset, Notional Amount	0.0	(581.2)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | TBA securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	72.9	3.7
Derivative Liability, Fair Value, Gross Liability	1.5	0.6
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | Forward settling securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	5.6	0.0
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | Over the Counter (OTC) Commodity Contracts [Member]
Derivative Asset, Fair Value, Gross Asset	0.0	5.2
Payable to broker-dealers, clearing organizations and counterparties [Member] | TBA securities sold [Member]
Derivative Asset, Fair Value, Gross Asset	2.0	0.9
Derivative Liability, Fair Value, Gross Liability	94.8	5.9
Payable to broker-dealers, clearing organizations and counterparties [Member] | Over the Counter (OTC) Commodity Contracts [Member]
Derivative Liability, Notional Amount	(712.4)	0.0
Derivative Liability, Fair Value, Gross Liability	$ 2.0	$ 0.3