Investments - Allowance for Credit Losses (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	$ (9.6)	$ 7.5
Additions for Securities for which No Previous Expected Credit Losses were Recognized	2.2	(5.6)
Reduction Due to Sales	(2.2)
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(1.0)	(5.1)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	(8.6)	7.5
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff	0.0	(10.7)
Equity securities, FV-NI, unrealized gain (loss)	5.1
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	0.0	0.0
Additions for Securities for which No Previous Expected Credit Losses were Recognized		(0.4)
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized		0.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss	0.0	0.4
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff		0.0
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	(8.9)	7.5
Additions for Securities for which No Previous Expected Credit Losses were Recognized	2.2	(5.2)
Reduction Due to Sales	(1.9)
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(1.1)	(5.1)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	(8.1)	7.1
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff	0.0	$ (10.7)
States and Political Subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	(0.7)
Additions for Securities for which No Previous Expected Credit Losses were Recognized	0.0
Reduction Due to Sales	(0.3)
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	0.1
Debt Securities, Available-for-Sale, Allowance for Credit Loss	(0.5)
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff	$ 0.0

Amount of allowance for credit loss on 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
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
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-Section 50
-Paragraph 9
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-9

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
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- Definition

Amount of expense for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), for which no credit loss was previously recorded.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
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-Section 50
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- Definition

Amount of expense (reversal of expense) for allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), not intended to be sold and not more likely than not will be required to sell before recovery of amortized cost basis, with previously recorded allowance.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-9

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

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Amount of decrease in allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) from sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
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Amount of writeoff of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
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-Section 50
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- Definition

Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
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