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Investments - Amortized Cost, Allowance for Credit Losses, Gross Unrealized Gains and Losses, and Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	$ 8,956.7	$ 8,635.3
Fixed maturity securities available for sale, allowances for credit losses	(2.8)	(1.9)
Fixed maturity securities, gross unrealized gains	96.7	174.9
Fixed maturity securities, gross unrealized losses	(266.8)	(230.6)
Fixed maturity securities, fair value	8,783.8	8,577.7
U.S. government and government agencies and authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	63.7	62.7
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	0.5	1.0
Fixed maturity securities, gross unrealized losses	(1.1)	(0.8)
Fixed maturity securities, fair value	63.1	62.9
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	99.2	100.0
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	0.8	1.2
Fixed maturity securities, gross unrealized losses	(5.2)	(5.1)
Fixed maturity securities, fair value	94.8	96.1
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	625.2	596.1
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	5.1	9.1
Fixed maturity securities, gross unrealized losses	(15.3)	(11.3)
Fixed maturity securities, fair value	615.0	593.9
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	856.3	850.2
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	3.3	4.6
Fixed maturity securities, gross unrealized losses	(9.5)	(9.1)
Fixed maturity securities, fair value	850.1	845.7
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	434.4	431.4
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	3.3	5.3
Fixed maturity securities, gross unrealized losses	(18.7)	(19.3)
Fixed maturity securities, fair value	419.0	417.4
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	1,032.2	978.6
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	6.8	11.5
Fixed maturity securities, gross unrealized losses	(37.5)	(35.4)
Fixed maturity securities, fair value	1,001.5	954.7
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	4,055.3	3,895.5
Fixed maturity securities available for sale, allowances for credit losses	(2.8)	(1.9)
Fixed maturity securities, gross unrealized gains	51.6	97.3
Fixed maturity securities, gross unrealized losses	(133.6)	(113.3)
Fixed maturity securities, fair value	3,970.5	3,877.6
Foreign corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, cost or amortized cost	1,790.4	1,720.8
Fixed maturity securities available for sale, allowances for credit losses	0.0	0.0
Fixed maturity securities, gross unrealized gains	25.3	44.9
Fixed maturity securities, gross unrealized losses	(45.9)	(36.3)
Fixed maturity securities, fair value	$ 1,769.8	$ 1,729.4

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) 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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