Investments - Schedule of Available-for-sale Investments (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Debt Securities, Available-for-sale [Line Items]
Available-for-sale investments	$ 43.9	$ 419.0
Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Available-for-sale investments	0.6	169.6
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale investments	31.8	158.4
Government securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale investments	6.3	65.8
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale investments	$ 5.2	$ 25.2

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant