Investments - Schedule of Investments (Details) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2017	Sep. 30, 2017
Investment [Line Items]
Total	$ 1,374.6	$ 1,387.6
Gross unrealized losses on available-for-sale investments	1.5
Certificates of deposit and time deposits [Member]
Investment [Line Items]
Total	975.1	1,005.3
Commercial paper [Member]
Investment [Line Items]
Total	26.4	20.3
Corporate debt securities [Member]
Investment [Line Items]
Total	201.0	199.4
Government securities [Member]
Investment [Line Items]
Total	123.9	116.8
Asset-backed Securities [Member]
Investment [Line Items]
Total	$ 48.2	$ 45.8

Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Value of the investment at close of period. For schedules of investments that are categorized, the value would be aggregated by category. For investment in and advances to affiliates, if operations of any controlled companies are different in character from those of the company, group such affiliates within divisions and by type of activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 210
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=63488139&loc=d3e12121-115841

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611133-123010

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 6
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 946
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Reference 5: http://www.xbrl.org/2003/role/presentationRef
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-Section S99
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