Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018
Derivative assets:
Total derivative assets — gross	$ 83.9	$ 232.5	$ 147.6
Gross amounts offset in the balance sheet	(25.5)	(34.3)	(29.3)
Cash collateral received	0.0	(12.2)	(1.5)
Total derivative assets — net	58.4	186.0	116.8
Derivative liabilities:
Total derivative liabilities — gross	(120.2)	(58.8)	(53.6)
Gross amounts offset in the balance sheet	25.5	34.3	29.3
Cash collateral pledged	2.3	0.0		$ 0.0
Total derivative liabilities — net	(92.4)	(24.5)	(24.3)
Derivatives subject to PGC and DS mechanisms:
Derivative assets:
Total derivative assets — gross	1.0	3.0	2.0
Derivative liabilities:
Total derivative liabilities — gross	(3.1)	(0.1)	(0.1)
Derivatives designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	7.3	2.4	1.8
Derivative liabilities:
Total derivative liabilities — gross	(7.8)	(1.4)	(2.3)
Derivatives designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	7.3	1.5	1.1
Derivative liabilities:
Total derivative liabilities — gross	0.0	(0.4)	(0.8)
Derivatives designated as hedging instruments: | Cross-currency contracts
Derivative assets:
Total derivative assets — gross	0.0	0.9	0.7
Derivatives designated as hedging instruments: | Interest rate contracts
Derivative liabilities:
Total derivative liabilities — gross	(7.8)	(1.0)	(1.5)
Derivatives not designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	75.6	227.1	143.8
Derivative liabilities:
Total derivative liabilities — gross	(109.3)	(57.3)	(51.2)
Derivatives not designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	22.2	19.1	17.3
Derivative liabilities:
Total derivative liabilities — gross	(5.2)	(14.0)	(17.1)
Derivatives not designated as hedging instruments: | Commodity contracts
Derivative assets:
Total derivative assets — gross	53.4	208.0	126.5
Derivative liabilities:
Total derivative liabilities — gross	$ (104.1)	$ (43.3)	$ (34.1)