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, 2019	Sep. 30, 2018
Derivative Asset, Fair Value, Gross Asset	$ 4,044.2	$ 8,035.9
Derivative Liability, Fair Value, Gross Liability	4,019.0	8,511.0
TBA securities purchased [Member]
Derivative Liability, Notional Amount	(451.8)	(293.2)
Derivative Asset, Notional Amount	(1,397.6)	(721.5)
TBA securities sold [Member]
Derivative Liability, Notional Amount	(2,260.8)	(812.7)
Derivative Asset, Notional Amount	(425.4)	(1,099.5)
Forward settling securities purchased [Member]
Derivative Asset, Notional Amount	(1,258.1)	(614.3)
Over the Counter (OTC) Commodity Contracts [Member]
Derivative Liability, Notional Amount	(1,172.7)	0.0
Derivative Asset, Notional Amount	0.0	(427.2)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Domain] | TBA securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	7.1	1.2
Derivative Liability, Fair Value, Gross Liability	(0.4)	(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	4.5	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	0.0	0.1
Payable to broker-dealers, clearing organizations and counterparties [Member] | TBA securities sold [Member]
Derivative Asset, Fair Value, Gross Asset	0.5	0.0
Derivative Liability, Fair Value, Gross Liability	(13.7)	0.0
Payable to broker-dealers, clearing organizations and counterparties [Member] | Over the Counter (OTC) Commodity Contracts [Member]
Derivative Liability, Fair Value, Gross Liability	$ (2.6)	$ 0.0