Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017
Derivative assets:
Total derivative assets — gross	$ 80.5	$ 116.3	$ 92.6
Gross amounts offset in the balance sheet	(31.5)	(35.7)	(31.8)
Cash collateral received	(0.4)	(8.3)	(0.2)
Total derivative assets — net	48.6	72.3	60.6
Derivative liabilities:
Total derivative liabilities — gross	(85.4)	(82.5)	(42.7)
Gross amounts offset in the balance sheet	31.5	35.7	31.8
Total derivative liabilities — net	(53.9)	(46.8)	(10.9)
Derivatives designated as hedging instruments
Derivative assets:
Total derivative assets — gross	0.3	3.2	17.2
Derivative liabilities:
Total derivative liabilities — gross	(8.1)	(10.7)	(2.2)
Derivatives designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	0.3	3.2	14.2
Derivative liabilities:
Total derivative liabilities — gross	(4.0)	(5.5)	0.0
Derivatives designated as hedging instruments | Cross-currency contracts
Derivative assets:
Total derivative assets — gross	0.0	0.0	3.0
Derivative liabilities:
Total derivative liabilities — gross	(2.2)	(2.9)	0.0
Derivatives designated as hedging instruments | Interest rate contracts
Derivative liabilities:
Total derivative liabilities — gross	(1.9)	(2.3)	(2.2)
Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Total derivative assets — gross	0.9	1.7	2.1
Derivative liabilities:
Total derivative liabilities — gross	(0.6)	(1.5)	(0.2)
Derivatives not designated as hedging instruments
Derivative assets:
Total derivative assets — gross	79.3	111.4	73.3
Derivative liabilities:
Total derivative liabilities — gross	(76.7)	(70.3)	(40.3)
Derivatives not designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	13.6	9.0	1.4
Derivative liabilities:
Total derivative liabilities — gross	(39.5)	(32.7)	(1.5)
Derivatives not designated as hedging instruments | Commodity contracts
Derivative assets:
Total derivative assets — gross	65.7	102.4	71.9
Derivative liabilities:
Total derivative liabilities — gross	$ (37.2)	$ (37.6)	$ (38.8)