Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016
Derivative assets:
Total derivative assets — gross	$ 51.1	$ 72.7	$ 89.1
Gross amounts offset in the balance sheet	(34.6)	(35.0)	(36.8)
Cash collateral received	(0.1)	(0.3)	(2.3)
Total derivative assets — net	16.4	37.4	50.0
Derivative liabilities:
Total derivative liabilities — gross	(70.3)	(105.4)	(76.0)
Gross amounts offset in the balance sheet	34.6	35.0	36.8
Total derivative liabilities — net	(35.7)	(70.4)	(39.2)
Derivatives designated as hedging instruments
Derivative assets:
Total derivative assets — gross	5.2	17.8	18.9
Derivative liabilities:
Total derivative liabilities — gross	(5.2)	(6.8)	(4.6)
Derivatives designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	5.2	17.8	18.6
Derivative liabilities:
Total derivative liabilities — gross	(2.1)	(2.4)	(0.8)
Derivatives designated as hedging instruments | Cross-currency contracts
Derivative assets:
Total derivative assets — gross	0.0	0.0	0.3
Derivative liabilities:
Total derivative liabilities — gross	(0.9)	(0.5)	0.0
Derivatives designated as hedging instruments | Interest rate contracts
Derivative liabilities:
Total derivative liabilities — gross	(2.2)	(3.9)	(3.8)
Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Total derivative assets — gross	1.2	4.5	5.7
Derivative liabilities:
Total derivative liabilities — gross	(1.2)	(0.5)	(0.6)
Derivatives not designated as hedging instruments
Derivative assets:
Total derivative assets — gross	44.7	50.4	64.5
Derivative liabilities:
Total derivative liabilities — gross	(63.9)	(98.1)	(70.8)
Derivatives not designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	6.1	0.0	0.0
Derivative liabilities:
Total derivative liabilities — gross	(22.2)	0.0	0.0
Derivatives not designated as hedging instruments | Commodity contracts
Derivative assets:
Total derivative assets — gross	38.6	50.4	64.5
Derivative liabilities:
Total derivative liabilities — gross	$ (41.7)	$ (98.1)	$ (70.8)