Investments - Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2023	Mar. 31, 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	0.3	(3.8)
Net Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(0.6)	(1.3)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	(7.4)	9.1
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff	0.0	(3.5)
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	0.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss	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	0.3	(3.8)
Net Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(0.7)	(1.3)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	(6.6)	9.1
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff	0.0	$ (3.5)
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
Net Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	0.1
Debt Securities, Available-for-Sale, Allowance for Credit Loss	(0.8)
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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=124269663&loc=SL82922900-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (i)
-URI https://asc.fasb.org/extlink&oid=124269663&loc=SL82922900-210455

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

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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.

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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.

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- Definition

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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- Definition

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).

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