Fair Value Measurement - Offsetting Derivative Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Sep. 30, 2013	Mar. 31, 2013
Fair Value Disclosures [Abstract]
Derivative Asset, Fair Value, Gross Asset	$ 31.6	$ 26.3	$ 30.1
Derivative Asset, Fair Value, Offset Amount	(8.6)	(2.1)	(10.8)
Derivative Asset, Fair Value, Net Amount	23.0	24.2	19.3
Derivative Liability, Fair Value, Gross Liability	(65.3)	(57.5)	(87.1)
Derivative Liability, Fair Value, Offset Amount	8.6	2.1	10.8
Derivative Liability, Fair Value, Net Amount	(56.7)	(55.4)	(76.3)
Carrying value long-term debt	3,613.6		3,502.9
Long-term Debt, Fair Value	$ 3,864.4		$ 3,780.6