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	Jun. 30, 2020	Sep. 30, 2019
Derivative Asset, Fair Value, Gross Asset	$ 3,875.2	$ 3,671.5
Derivative Liability, Fair Value, Gross Liability	(3,892.8)	(3,604.5)
TBA securities purchased [Member]
Derivative Liability, Notional Amount	(928.5)	(234.5)
Derivative Asset, Notional Amount	(2,170.9)	(1,778.4)
TBA securities sold [Member]
Derivative Liability, Notional Amount	(2,982.1)	(2,788.0)
Derivative Asset, Notional Amount	(1,566.8)	(451.6)
Forward settling securities purchased [Member]
Derivative Asset, Notional Amount	(1,690.5)	0.0
Over the Counter (OTC) Commodity Contracts [Member]
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	7.7	3.7
Derivative Liability, Fair Value, Gross Liability	1.9	0.6
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | Forward settling securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	1.1	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	3.4	0.9
Derivative Liability, Fair Value, Gross Liability	10.2	5.9
Payable to broker-dealers, clearing organizations and counterparties [Member] | Forward settling securities purchased [Member]
Derivative Liability, Notional Amount	0.0	(1,243.5)
Derivative Liability, Fair Value, Gross Liability	0.0	0.3
Payable to broker-dealers, clearing organizations and counterparties [Member] | Over the Counter (OTC) Commodity Contracts [Member]
Derivative Liability, Notional Amount	(1,268.3)	0.0
Derivative Liability, Fair Value, Gross Liability	$ 0.5	$ 0.0