Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017
Derivative assets:
Total derivative assets — gross	$ 147.6	$ 116.3	$ 51.1
Gross amounts offset in the balance sheet	(29.3)	(35.7)	(34.6)
Cash collateral received	(1.5)	(8.3)	(0.1)
Total derivative assets — net	116.8	72.3	16.4
Derivative liabilities:
Total derivative liabilities — gross	(53.6)	(82.5)	(70.3)
Gross amounts offset in the balance sheet	29.3	35.7	34.6
Total derivative liabilities — net	(24.3)	(46.8)	(35.7)
Derivatives designated as hedging instruments
Derivative assets:
Total derivative assets — gross	1.8	3.2	5.2
Derivative liabilities:
Total derivative liabilities — gross	(2.3)	(10.7)	(5.2)
Derivatives designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	1.1	3.2	5.2
Derivative liabilities:
Total derivative liabilities — gross	(0.8)	(5.5)	(2.1)
Derivatives designated as hedging instruments | Cross-currency contracts
Derivative assets:
Total derivative assets — gross	0.7	0.0	0.0
Derivative liabilities:
Total derivative liabilities — gross	0.0	(2.9)	(0.9)
Derivatives designated as hedging instruments | Interest rate contracts
Derivative liabilities:
Total derivative liabilities — gross	(1.5)	(2.3)	(2.2)
Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Total derivative assets — gross	2.0	1.7	1.2
Derivative liabilities:
Total derivative liabilities — gross	(0.1)	(1.5)	(1.2)
Derivatives not designated as hedging instruments
Derivative assets:
Total derivative assets — gross	143.8	111.4	44.7
Derivative liabilities:
Total derivative liabilities — gross	(51.2)	(70.3)	(63.9)
Derivatives not designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	17.3	9.0	6.1
Derivative liabilities:
Total derivative liabilities — gross	(17.1)	(32.7)	(22.2)
Derivatives not designated as hedging instruments | Commodity contracts
Derivative assets:
Total derivative assets — gross	126.5	102.4	38.6
Derivative liabilities:
Total derivative liabilities — gross	$ (34.1)	$ (37.6)	$ (41.7)