Investments - Net Investment Gains (Losses) by Transaction Type (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Investments, Debt and Equity Securities [Abstract]
Credit loss impairments	$ (4.0)	$ 1.0	$ (4.0)	$ (1.2)
Intent-to-sell impairments	(0.1)	(0.7)	(0.2)	(1.6)
Total impairments	(4.1)	0.3	(4.2)	(2.8)
Sales and other, net	(14.7)	0.3	(8.5)	4.4
Change in fair value - equity securities	(2.8)	2.2	(4.4)	0.3
Change in fair value and gains (losses) realized on settlements - derivatives	(4.6)	0.5	(11.4)	(7.8)
Net investment gains (losses)	$ (26.2)	$ 3.3	$ (28.5)	$ (5.9)

Impairments Of Investments

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Other Than Temporary Impairment Loss, Investments, Portion Recognized In Earnings, Change In Intent

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- Definition

Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-Name Accounting Standards Codification
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Amount of unrealized and realized gain (loss) on investment in debt and equity securities.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
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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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Amount of unrealized and realized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of realized gain (loss) on investment.

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