Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019
Derivative assets:
Total derivative assets — gross	$ 116.6	$ 101.1	$ 83.9
Gross amounts offset in the balance sheet	(42.4)	(29.0)	(25.5)
Total derivative assets — net	74.2	72.1	58.4
Derivative liabilities:
Total derivative liabilities — gross	(237.9)	(191.6)	(120.2)
Gross amounts offset in the balance sheet	42.4	29.0	25.5
Cash collateral pledged	20.7	29.3	2.3
Total derivative liabilities — net	(174.8)	(133.3)	(92.4)
Derivatives subject to PGC and DS mechanisms:
Derivative assets:
Total derivative assets — gross	2.6	1.4	1.0
Derivative liabilities:
Total derivative liabilities — gross	(1.6)	(3.7)	(3.1)
Derivatives designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	24.8	17.4	7.3
Derivatives designated as hedging instruments: | Interest rate contracts
Derivative liabilities:
Total derivative liabilities — gross	(59.0)	(12.3)	(7.8)
Derivatives not designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	89.2	82.3	75.6
Derivative liabilities:
Total derivative liabilities — gross	(177.3)	(175.6)	(109.3)
Derivatives not designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	24.2	41.6	22.2
Derivative liabilities:
Total derivative liabilities — gross	(7.0)	(4.3)	(5.2)
Derivatives not designated as hedging instruments: | Commodity contracts
Derivative assets:
Total derivative assets — gross	65.0	40.7	53.4
Derivative liabilities:
Total derivative liabilities — gross	$ (170.3)	$ (171.3)	$ (104.1)