Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Realized Gains/Losses on Derivative Contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ 47.3	$ 41.8	$ 78.6
Foreign Exchange Forward [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	8.7	9.7	7.5
Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.1)	0.8	3.2
TBA and forward settling securities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(2.5)	(14.4)	(5.1)
Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ 53.4	$ 37.9	$ 84.2