Investments - Allowance for Credit Loss Impairments on Fixed Maturity Securities (Details) - Fixed maturity securities - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-Sale, Excluding Accrued Interest, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 1.2	$ 7.7	$ 0.0
Credit losses on fixed maturity securities for which credit losses were not previously reported	0.0	0.0	8.1
Net increases (decreases) related to credit losses previously reported	0.0	3.1	0.0
Reduction of credit allowances related to sales	0.0	(9.2)	0.0
Write-offs	0.0	(0.4)	(0.4)
Ending balance	$ 1.2	$ 1.2	$ 7.7

Amount excluding accrued interest, 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 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

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Balance Type:	credit
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- Definition

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

+ References

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

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- Definition

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

+ References

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

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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.

+ References

No definition available.

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- Definition

Amount, excluding accrued interest, 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
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-9

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- Definition

+ References

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

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