Investments Investments, Net Realized Gains and Losses (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Realized Investment Gains (Losses)
Total gross realized gains	$ 19.1	$ 2.6	$ 21.2	$ 3.4
Investments, Gross Realized Losses	(1.2)	(1.6)	(2.1)	(3.2)
Allowance for Credit Loss
Investments, Net (Increase)/Decrease in CECL Allowance	(0.4)	0.3	(0.7)
Debt and Equity Securities, Unrealized Gain (Loss)
Change in net unrealized gains (losses)	(2.9)	29.1	(26.2)	45.1
Net realized and unrealized gains on investments	18.7	20.9	17.0	8.1
AFS, Change in Unrealized Gains (Losses) on Fixed Maturity and Equity Securities reflected in AOCI	(4.1)	9.5	(24.8)	37.2
Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale
Proceeds from sale of fixed maturity securities	31.7	28.7	79.0	75.8
Debt Securities [Member]
Realized Investment Gains (Losses)
Debt Securities, Available-for-sale, Realized Gain	0.2	0.1	0.6	0.2
Debt Securities, Available-for-sale, Realized Loss	(0.3)	(0.5)	(0.9)	(1.2)
Allowance for Credit Loss
Investments, Net (Increase)/Decrease in CECL Allowance	(0.4)	0.3	(0.7)
Debt and Equity Securities, Unrealized Gain (Loss)
Change in net unrealized gains (losses)	(4.1)	9.5	(24.8)	37.2
Net realized and unrealized gains on investments	(0.5)	(0.1)	(1.0)	(1.0)
AFS, Change in Unrealized Gains (Losses) on Fixed Maturity and Equity Securities reflected in AOCI	(4.1)	9.5	(24.8)	37.2
Equity Securities [Member]
Realized Investment Gains (Losses)
Equity Securities, FV-NI, Realized Gain	18.9	2.5	20.6	3.2
Equity Securities, FV-NI, Realized Loss	(0.9)	(1.1)	(1.2)	(2.0)
Allowance for Credit Loss
Investments, Net (Increase)/Decrease in CECL Allowance	0.0	0.0	0.0	0.0
Debt and Equity Securities, Unrealized Gain (Loss)
Change in net unrealized gains (losses)	(0.3)	17.3	(3.0)	11.0
Net realized and unrealized gains on investments	17.7	18.7	16.4	12.2
AFS, Change in Unrealized Gains (Losses) on Fixed Maturity and Equity Securities reflected in AOCI	0.0	0.0	0.0	0.0
Other Security Investments
Realized Investment Gains (Losses)
Gain on Sale of Investments	0.0	0.0	0.0	0.0
Loss on Sale of Investments	0.0	0.0	0.0	0.0
Allowance for Credit Loss
Investments, Net (Increase)/Decrease in CECL Allowance	0.0	0.0	0.0	0.0
Debt and Equity Securities, Unrealized Gain (Loss)
Change in net unrealized gains (losses)	1.5	2.3	1.6	(3.1)
Net realized and unrealized gains on investments	1.5	2.3	1.6	(3.1)
AFS, Change in Unrealized Gains (Losses) on Fixed Maturity and Equity Securities reflected in AOCI	$ 0.0	$ 0.0	$ 0.0	$ 0.0

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Total gross realized gains for all fixed maturity and equity securities.

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Total gross realized losses for all fixed maturity and equity securities.

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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).

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Amount of realized 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 gain from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of realized loss from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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

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The difference between the carrying value and the sale price of an investment. A gain would be recognized when the sale price of the investment is greater than the carrying value of the investment. This element refers to the Gain included in earnings and not to the cash proceeds of the sale.

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The difference between the carrying value and the sale price of an investment. A loss would be recognized when the sale price of the investment is less than the carrying value of the investment. This element refers to the Loss included in earnings and not to the cash proceeds of the sale.

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Amount, before tax and after adjustment, of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and unrealized gain (loss) on investment in debt security measured at amortized cost (held-to-maturity) from transfer to available-for-sale.

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Amount of cash inflow from sale 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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