Investments - Net Realized Gains on Sales of Investments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Net Investment Income [Line Items]
Net Realized Investment (Losses) Gains	$ (0.1)	$ 1.5	$ 0.8	$ 8.1
Fixed Maturities
Net Investment Income [Line Items]
Debt Securities, Available-for-Sale, Realized Gain	0.4	2.2	1.7	15.0
Debt Securities, Available-for-Sale, Realized Loss	(0.5)	(0.3)	(1.0)	(2.6)
Derivative
Net Investment Income [Line Items]
Gain (Loss) on Hedging Activity	0.0	0.0	0.0	(7.9)
Equity Securities
Net Investment Income [Line Items]
Debt Securities, Available-for-Sale, Realized Gain	0.0	0.0	0.0	4.1
Debt Securities, Available-for-Sale, Realized Loss	0.0	0.0	0.0	(0.1)
Other Investments
Net Investment Income [Line Items]
Debt Securities, Available-for-Sale, Realized Gain	0.0	1.5	0.1	1.5
Debt Securities, Available-for-Sale, Realized Loss	$ 0.0	$ (1.9)	$ 0.0	$ (1.9)

Amount of realized gain 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 9
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-9

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Amount of realized gain (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/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-9

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Gain (loss) incurred in hedging activities.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1

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