Investments - Schedule of Fair Value of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	$ 0.6	$ 43.9
Equity securities	875.3	721.5
Total investments	954.1	833.5
Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.6	0.6
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0	31.8
Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0	6.3
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.0	5.2
Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Equity securities	875.3	721.5
Total investments	875.9	765.4
Fair Value, Measurements, Recurring \| Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.6	0.6
Fair Value, Measurements, Recurring \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities		31.8
Fair Value, Measurements, Recurring \| Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities		6.3
Fair Value, Measurements, Recurring \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities		5.2
Fair Value, Measurements, Recurring \| Level 1
Debt Securities, Available-for-sale [Line Items]
Equity securities		721.5
Total investments	875.3	727.8
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
Fair Value, Measurements, Recurring \| Level 1 \| Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities		6.3
Fair Value, Measurements, Recurring \| Level 1 \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities		0.0
Fair Value, Measurements, Recurring \| Level 2
Debt Securities, Available-for-sale [Line Items]
Equity securities	0.0	0.0
Total investments	0.6	37.6
Fair Value, Measurements, Recurring \| Level 2 \| Certificates of deposit and time deposits
Debt Securities, Available-for-sale [Line Items]
Fixed income securities	0.6	0.6
Fair Value, Measurements, Recurring \| Level 2 \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities		31.8
Fair Value, Measurements, Recurring \| Level 2 \| Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities		0.0
Fair Value, Measurements, Recurring \| Level 2 \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities		5.2
Fair Value, Measurements, Recurring \| Level 3
Debt Securities, Available-for-sale [Line Items]
Equity securities	0.0
Total investments	0.0	0.0
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
Fair Value, Measurements, Recurring \| Level 3 \| Government securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities		0.0
Fair Value, Measurements, Recurring \| Level 3 \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed income securities		$ 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 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: 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=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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