Investments - Fair Values of Investments (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017
Assets
Total assets	$ 734.0	$ 1,387.6
Other current liabilities
Total liabilities	76.8
Certificates of deposit and time deposits [Member]
Assets
Available-for-sale securities	477.9	1,005.3
Commercial paper [Member]
Assets
Available-for-sale securities	0.0	20.3
Corporate debt securities [Member]
Assets
Available-for-sale securities	160.3	199.4
Government securities [Member]
Assets
Available-for-sale securities	65.8	116.8
Asset-backed securities [Member]
Assets
Available-for-sale securities	30.0	45.8
Fair Value, Inputs, Level 1 [Member]
Assets
Total assets	55.7	98.9
Other current liabilities
Total liabilities	0.0
Fair Value, Inputs, Level 1 [Member] \| Certificates of deposit and time deposits [Member]
Assets
Available-for-sale securities	0.0	0.0
Fair Value, Inputs, Level 1 [Member] \| Commercial paper [Member]
Assets
Available-for-sale securities	0.0	0.0
Fair Value, Inputs, Level 1 [Member] \| Corporate debt securities [Member]
Assets
Available-for-sale securities	0.0	0.0
Fair Value, Inputs, Level 1 [Member] \| Government securities [Member]
Assets
Available-for-sale securities	55.7	98.9
Fair Value, Inputs, Level 1 [Member] \| Asset-backed securities [Member]
Assets
Available-for-sale securities	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Assets
Total assets	678.3	1,288.7
Other current liabilities
Total liabilities	0.0
Fair Value, Inputs, Level 2 [Member] \| Certificates of deposit and time deposits [Member]
Assets
Available-for-sale securities	477.9	1,005.3
Fair Value, Inputs, Level 2 [Member] \| Commercial paper [Member]
Assets
Available-for-sale securities	0.0	20.3
Fair Value, Inputs, Level 2 [Member] \| Corporate debt securities [Member]
Assets
Available-for-sale securities	160.3	199.4
Fair Value, Inputs, Level 2 [Member] \| Government securities [Member]
Assets
Available-for-sale securities	10.1	17.9
Fair Value, Inputs, Level 2 [Member] \| Asset-backed securities [Member]
Assets
Available-for-sale securities	30.0	45.8
Fair Value, Inputs, Level 3 [Member]
Assets
Total assets	0.0	0.0
Other current liabilities
Total liabilities	76.8
Fair Value, Inputs, Level 3 [Member] \| Certificates of deposit and time deposits [Member]
Assets
Available-for-sale securities	0.0	0.0
Fair Value, Inputs, Level 3 [Member] \| Commercial paper [Member]
Assets
Available-for-sale securities	0.0	0.0
Fair Value, Inputs, Level 3 [Member] \| Corporate debt securities [Member]
Assets
Available-for-sale securities	0.0	0.0
Fair Value, Inputs, Level 3 [Member] \| Government securities [Member]
Assets
Available-for-sale securities	0.0	0.0
Fair Value, Inputs, Level 3 [Member] \| Asset-backed securities [Member]
Assets
Available-for-sale securities	0.0	$ 0.0
Forward Contracts [Member]
Other current liabilities
Forward contract	76.8
Forward Contracts [Member] \| Fair Value, Inputs, Level 1 [Member]
Other current liabilities
Forward contract	0.0
Forward Contracts [Member] \| Fair Value, Inputs, Level 2 [Member]
Other current liabilities
Forward contract	0.0
Forward Contracts [Member] \| Fair Value, Inputs, Level 3 [Member]
Other current liabilities
Forward contract	$ 76.8

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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Fair value of financial and nonfinancial obligations.

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