Investments - Schedule of Components of Net Investment Return (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]
Net interest and dividend income	$ 45.8	$ 47.9	$ 137.4	$ 146.2
Investment expenses	(1.2)	(2.0)	(5.1)	(6.2)
Net investment income	44.6	45.9	132.3	140.0
Net realized gains/(losses) on fixed maturity securities, available-for-sale	(2.7)	1.3	(3.4)	2.9
Net realized gain on derivatives	0.0	0.0	0.1	0.0
Net realized and unrealized gains/(losses) on other investments	(9.0)	4.7	17.9	10.4
Change in net unrealized losses on derivatives	0.0	(0.1)	0.0	(0.1)
Change in provision for expected credit losses	(15.7)	0.3	(18.7)	5.6
Net realized and unrealized investment gains/(losses)	(27.4)	6.2	(4.1)	18.8
Total realized and unrealized investment gains/(losses) and net investment income	$ 17.2	$ 52.1	$ 128.2	$ 158.8

Debt Securities, Loss On Derivatives, Unrealized Gain (Loss)

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Debt Securities, Other Investments, Unrealized Gain (Loss)

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Realized Gain (Loss) on Derivatives

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Amount of increase (decrease) in allowance for credit loss of 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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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).

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Amount of realized and unrealized gain (loss) on investment.

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Amount before accretion (amortization) of purchase discount (premium) of interest income and dividend income on nonoperating securities.

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Amount of expenses related to the generation of investment income.

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Amount after accretion (amortization) of discount (premium), and investment expense, of interest income and dividend income on nonoperating securities.

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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