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, 2018	Sep. 30, 2017
Derivative Asset, Fair Value, Gross Asset	$ 4,505.3	$ 4,321.2
Derivative Liability, Fair Value, Gross Liability	4,674.4	4,201.0
TBA securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	2.9	0.0
Derivative Liability, Notional Amount	(49.9)	(1,236.8)
Derivative Liability, Fair Value, Gross Liability	0.0	(2.9)
Derivative Asset, Notional Amount	832.5	51.3
TBA securities sold [Member]
Derivative Asset, Fair Value, Gross Asset	0.1	5.8
Derivative Liability, Notional Amount	(1,611.1)	(404.1)
Derivative Liability, Fair Value, Gross Liability	(5.8)	(0.1)
Derivative Asset, Notional Amount	118.7	1,881.9
Forward settling securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	0.3	(2.0)
Derivative Asset, Notional Amount	410.9	882.9
Over the Counter (OTC) Commodity Contracts [Member]
Derivative Liability, Notional Amount	(308.1)	(590.2)
Derivative Liability, Fair Value, Gross Liability	$ 1.8	$ 3.0