Investments - Net Investment Gains (Losses) by Transaction Type (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Investments, Debt and Equity Securities [Abstract]
Credit loss impairments	$ 0.0	$ (0.9)
Intent-to-sell impairments	0.0	0.0
Total impairments	0.0	(0.9)
Sales and other, net	0.2	(0.1)
Change in fair value - equity securities	(1.2)	2.6
Change in fair value and gains (losses) realized on settlements - derivatives	(2.3)	0.6
Net investment gains (losses)	$ (3.3)	$ 2.2

Impairments Of Investments

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- Definition

Other than Temporary Impairment Losses, Investments, Portion Recognized In Earnings, Change In Intent

+ References

No definition available.

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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://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

Amount of unrealized and realized gain (loss) on investment in debt and equity securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
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-Name Accounting Standards Codification
-Section 50
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-Publisher FASB
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- Definition

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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
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Amount of realized gain (loss) on investment.

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