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, 2015	Sep. 30, 2014
Derivative Asset, Fair Value, Gross Asset	$ 4,837.8	$ 6,606.8
Derivative Liability, Fair Value, Gross Liability	5,163.9	6,054.5
TBA securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	0.5
Derivative Liability, Notional Amount	(393.3)
Derivative Liability, Fair Value, Gross Liability	(0.5)
Derivative Asset, Notional Amount	(455.0)
TBA securities sold [Member]
Derivative Asset, Fair Value, Gross Asset	0.9
Derivative Liability, Notional Amount	(769.7)
Derivative Liability, Fair Value, Gross Liability	(1.0)
Derivative Asset, Notional Amount	(599.2)
Forward settling securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	0.1
Derivative Asset, Notional Amount	(139.8)
Over the Counter (OTC) Commodity Contracts [Member]
Derivative Asset, Fair Value, Gross Asset	0.1
Derivative Asset, Notional Amount	(137.9)
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivative Asset, Fair Value, Gross Asset	507.6	808.0
Derivative Liability, Fair Value, Gross Liability	$ 489.1	$ 741.8