Investments - Schedule of Fair Value of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	$ 43.9	$ 419.0
Equity securities	721.5	1,090.0
Total investments	833.5	1,578.9
Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.6	169.6
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	31.8	158.4
Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	6.3	65.8
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	5.2	25.2
Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	721.5	1,090.0
Total investments	765.4	1,509.0
Fair Value, Measurements, Recurring \| Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.6	169.6
Fair Value, Measurements, Recurring \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	31.8	158.4
Fair Value, Measurements, Recurring \| Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	6.3	65.8
Fair Value, Measurements, Recurring \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	5.2	25.2
Fair Value, Measurements, Recurring \| Level 1
Debt Securities, Available-for-sale [Line Items]
Equity securities	721.5	0.0
Total investments	727.8	55.7
Fair Value, Measurements, Recurring \| Level 1 \| Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0	0.0
Fair Value, Measurements, Recurring \| Level 1 \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0	0.0
Fair Value, Measurements, Recurring \| Level 1 \| Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	6.3	55.7
Fair Value, Measurements, Recurring \| Level 1 \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0	0.0
Fair Value, Measurements, Recurring \| Level 2
Debt Securities, Available-for-sale [Line Items]
Equity securities	0.0	0.0
Total investments	37.6	363.3
Fair Value, Measurements, Recurring \| Level 2 \| Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.6	169.6
Fair Value, Measurements, Recurring \| Level 2 \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	31.8	158.4
Fair Value, Measurements, Recurring \| Level 2 \| Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0	10.1
Fair Value, Measurements, Recurring \| Level 2 \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	5.2	25.2
Fair Value, Measurements, Recurring \| Level 3
Debt Securities, Available-for-sale [Line Items]
Equity securities	0.0	1,090.0
Total investments	0.0	1,090.0
Fair Value, Measurements, Recurring \| Level 3 \| Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0	0.0
Fair Value, Measurements, Recurring \| Level 3 \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0	0.0
Fair Value, Measurements, Recurring \| Level 3 \| Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0	0.0
Fair Value, Measurements, Recurring \| Level 3 \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	$ 0.0	$ 0.0

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

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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