Derivative Instruments and Hedging Activities - Schedule of Derivative Assets, Liabilities and the Effects of Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014
Derivatives, Fair Value
Derivative financial assets	$ 58.5	$ 45.4
Derivative asset offset amount, net	(18.9)	(18.4)
Total derivative assets - net	39.6	27.0
Derivative financial liabilities	(179.9)	(75.2)
Gross amounts offset in the balance sheet	18.9	18.4
Cash collateral pledged	8.0	0.0
Total derivative liabilities - net	(153.0)	(56.8)
Designated as Hedging Instruments
Derivatives, Fair Value
Derivative financial assets	29.5	17.8
Derivative financial liabilities	(10.9)	(26.4)
Designated as Hedging Instruments | Commodity Contracts
Derivatives, Fair Value
Derivative financial assets	0.0	2.8
Derivative financial liabilities	0.0	(5.3)
Designated as Hedging Instruments | Foreign Currency Contracts
Derivatives, Fair Value
Derivative financial assets	29.1	12.8
Derivative financial liabilities	(0.1)	(0.1)
Designated as Hedging Instruments | Interest Rate Contracts
Derivatives, Fair Value
Derivative financial assets	0.0	0.1
Derivative financial liabilities	(10.8)	(21.0)
Designated as Hedging Instruments | Cross Currency Contracts
Derivatives, Fair Value
Derivative financial assets	0.4	2.1
Derivative financial liabilities	0.0	0.0
Derivatives Subject To PGC and DS Mechanisms [Member] | Commodity Contracts
Derivatives, Fair Value
Derivative financial assets	1.3	1.7
Derivative financial liabilities	(5.6)	(2.2)
Not Designated as Hedging Instruments | Commodity Contracts
Derivatives, Fair Value
Derivative financial assets	27.7	25.9
Derivative financial liabilities	$ (163.4)	$ (46.6)