Derivative Instruments and Hedging Activities - Offsetting Derivative Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Document Period End Date	Mar. 31, 2015
Derivative asset, gross	$ 67.8	$ 31.6	[1]
Derivative asset, gross liability	(13.8)	(8.6)
Derivative asset, net	54.0	23.0
Cash collateral (received) pledged	0	0
Derivative asset, net amount recognized in balance sheet	54.0	23.0
Derivative liability, gross	(180.8)	(65.3)	[1]
Derivative liability, gross asset	13.8	8.6
Derivative liability, net	(167.0)	(56.7)
Cash collateral (received) pledged	3.7	0
Derivative liability, net amount recognized in balance sheet	$ (163.3)	$ (56.7)

[1] Certain immaterial amounts have been revised to correct the classification of derivatives.