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, unless otherwise specified	Mar. 31, 2015	Sep. 30, 2014
Derivative Asset, Fair Value, Gross Asset	$ 7,855.5	$ 6,606.8
Derivative Liability, Fair Value, Gross Liability	7,613.2	6,054.5
TBA securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	0.9
Derivative Liability, Notional Amount	(153.6)
Derivative Liability, Fair Value, Gross Liability	(0.2)
Derivative Asset, Notional Amount	(276.3)
TBA securities sold [Member]
Derivative Asset, Fair Value, Gross Asset	0.2
Derivative Liability, Notional Amount	(608.3)
Derivative Liability, Fair Value, Gross Liability	(2.4)
Derivative Asset, Notional Amount	(217.4)
Forward settling securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	0.3
Derivative Asset, Notional Amount	(206.6)
Over the Counter (OTC) Commodity Contracts [Member]
Derivative Asset, Fair Value, Gross Asset	0.6
Derivative Asset, Notional Amount	(254.7)
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivative Asset, Fair Value, Gross Asset	456.3	808.0
Derivative Liability, Fair Value, Gross Liability	$ 492.6	$ 741.8