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	Dec. 31, 2019	Sep. 30, 2019
Derivative Asset, Fair Value, Gross Asset	$ 2,115.1	$ 3,671.5
Derivative Liability, Fair Value, Gross Liability	2,091.6	3,604.5
TBA securities purchased [Member]
Derivative Liability, Notional Amount	174.1	234.5
Derivative Asset, Notional Amount	(1,283.0)	(1,778.4)
TBA securities sold [Member]
Derivative Liability, Notional Amount	2,182.9
Derivative Asset, Notional Amount	(280.1)
Over the Counter (OTC) Commodity Contracts [Member]
Derivative Liability, Notional Amount	406.7	1,243.5
Derivative Asset, Notional Amount	(294.4)	(581.2)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | TBA securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	2.5	3.7
Derivative Liability, Fair Value, Gross Liability	(0.1)	(0.6)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | Over the Counter (OTC) Commodity Contracts [Member]
Derivative Asset, Fair Value, Gross Asset	3.0	5.2
Payable to broker-dealers, clearing organizations and counterparties [Member] | TBA securities sold [Member]
Derivative Asset, Fair Value, Gross Asset	0.4	0.9
Derivative Liability, Fair Value, Gross Liability	(3.7)	(5.9)
Payable to broker-dealers, clearing organizations and counterparties [Member] | Over the Counter (OTC) Commodity Contracts [Member]
Derivative Liability, Fair Value, Gross Liability	$ (0.6)	$ (0.3)