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, 2019	Sep. 30, 2018
Derivative Asset, Fair Value, Gross Asset	$ 4,413.7	$ 8,035.9
Derivative Liability, Fair Value, Gross Liability	4,725.7	8,511.0
TBA securities purchased [Member]
Derivative Liability, Notional Amount	254.5	293.2
Derivative Asset, Notional Amount	(1,256.6)	(721.5)
TBA securities sold [Member]
Derivative Liability, Notional Amount	2,132.1	812.7
Derivative Asset, Notional Amount	(290.6)	(1,099.5)
Forward settling securities purchased [Member]
Derivative Asset, Notional Amount		(614.3)
Over the Counter (OTC) Commodity Contracts [Member]
Derivative Liability, Notional Amount	492.7	0.0
Derivative Asset, Notional Amount	(345.7)	(427.2)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | TBA securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	3.6	1.2
Derivative Liability, Fair Value, Gross Liability	(0.2)	(0.6)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | TBA securities sold [Member]
Derivative Asset, Fair Value, Gross Asset	0.0	3.2
Derivative Liability, Fair Value, Gross Liability	0.0	(1.5)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | Forward settling securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset		0.5
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | Over the Counter (OTC) Commodity Contracts [Member]
Derivative Asset, Fair Value, Gross Asset	2.2	0.1
Payable to broker-dealers, clearing organizations and counterparties [Member] | TBA securities sold [Member]
Derivative Asset, Fair Value, Gross Asset	0.3	0.0
Derivative Liability, Fair Value, Gross Liability	(6.9)	0.0
Payable to broker-dealers, clearing organizations and counterparties [Member] | Over the Counter (OTC) Commodity Contracts [Member]
Derivative Liability, Fair Value, Gross Liability	$ (1.3)	$ 0.0