Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019
Derivative assets:
Total derivative assets — gross	$ 106.3	$ 101.1	$ 90.8
Gross amounts offset in the balance sheet	(25.7)	(29.0)	(35.8)
Total derivative assets — net	80.6	72.1	55.0
Derivative liabilities:
Total derivative liabilities — gross	(374.7)	(191.6)	(83.3)
Gross amounts offset in the balance sheet	25.7	29.0	35.8
Cash collateral pledged	58.0	29.3	0.0
Total derivative liabilities — net	(291.0)	(133.3)	(47.5)
Derivatives subject to PGC and DS mechanisms:
Derivative assets:
Total derivative assets — gross	1.4	1.4	1.3
Derivative liabilities:
Total derivative liabilities — gross	(1.9)	(3.7)	(0.2)
Derivatives designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	27.2	17.4	7.1
Derivatives designated as hedging instruments: | Interest rate contracts
Derivative liabilities:
Total derivative liabilities — gross	(54.1)	(12.3)	(5.7)
Derivatives not designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	77.7	82.3	82.4
Derivative liabilities:
Total derivative liabilities — gross	(318.7)	(175.6)	(77.4)
Derivatives not designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	30.8	41.6	22.7
Derivative liabilities:
Total derivative liabilities — gross	(6.8)	(4.3)	(4.9)
Derivatives not designated as hedging instruments: | Commodity contracts
Derivative assets:
Total derivative assets — gross	46.9	40.7	59.7
Derivative liabilities:
Total derivative liabilities — gross	$ (311.9)	$ (171.3)	$ (72.5)