Financial Instruments, Risk Management and Fair Value Measurements - Fair Value of Derivatives by Balance Sheet Location (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Derivative Asset [Abstract]
Total gross amounts	$ 73.7	$ 8.3
Gross amounts offset in the condensed consolidated balance sheet	(8.2)	(8.1)
Net amounts	65.5	0.2
Derivative Liability [Abstract]
Total gross amounts	(32.5)	(17.2)
Gross amounts offset in the condensed consolidated balance sheet	8.2	8.1
Net amounts	(24.3)	(9.1)
Total gross amounts of derivative assets (liabilities)	41.2	(8.9)
Net amounts of derivative assets (liabilities)	41.2	(8.9)
Foreign exchange contracts
Derivative Asset [Abstract]
Total gross amounts	73.7	8.3
Gross amounts offset in the condensed consolidated balance sheet	(8.2)	(8.1)
Net amounts	65.5	0.2
Derivative Liability [Abstract]
Total gross amounts	(27.2)	(16.3)
Gross amounts offset in the condensed consolidated balance sheet	8.2	8.1
Net amounts	(19.0)	(8.2)
Interest rate contracts
Derivative Liability [Abstract]
Total gross amounts	(5.3)	(0.9)
Gross amounts offset in the condensed consolidated balance sheet	0.0	0.0
Net amounts	(5.3)	(0.9)
Designated as Hedging Instrument
Derivative Asset [Abstract]
Total gross amounts	52.4	8.0
Derivative Liability [Abstract]
Total gross amounts	(32.5)	(13.0)
Total gross amounts of derivative assets (liabilities)	19.9	(5.0)
Designated as Hedging Instrument \| Foreign exchange contracts
Derivative Asset [Abstract]
Total gross amounts	52.4	8.0
Derivative Liability [Abstract]
Total gross amounts	(27.2)	(12.1)
Designated as Hedging Instrument \| Interest rate contracts
Derivative Liability [Abstract]
Total gross amounts	(5.3)	(0.9)
Not Designated as Hedging Instrument
Derivative Asset [Abstract]
Total gross amounts	21.3	0.3
Derivative Liability [Abstract]
Total gross amounts	0.0	(4.2)
Total gross amounts of derivative assets (liabilities)	21.3	(3.9)
Not Designated as Hedging Instrument \| Foreign exchange contracts
Derivative Asset [Abstract]
Total gross amounts	21.3	0.3
Derivative Liability [Abstract]
Total gross amounts	0.0	(4.2)
Not Designated as Hedging Instrument \| Interest rate contracts
Derivative Liability [Abstract]
Total gross amounts	$ 0.0	$ 0.0

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that in accordance with the entity's accounting policy were offset against collateral under a master netting arrangement.

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, after offset of derivative liability, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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