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Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Net Investment Income [Line Items]
Investment income (loss)	$ 111.0	$ 117.9	$ 322.1	$ 324.9
Investment expenses	6.2	6.8	20.2	20.4
Net Investment Income	104.8	111.1	301.9	304.5
Fixed Maturities
Net Investment Income [Line Items]
Investment income (loss)	79.1	79.0	232.4	239.0
Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	2.1	0.9	3.6	4.6
Total Alternative Investments
Net Investment Income [Line Items]
Investment income (loss)	4.0	10.0	4.8	2.8
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income (loss)	0.0	0.9	(6.0)	(15.9)
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	4.0	9.1	10.8	18.7
Short-term Investments
Net Investment Income [Line Items]
Investment income (loss)	4.2	8.4	18.8	23.0
Loans to Policyholders
Net Investment Income [Line Items]
Investment income (loss)	5.2	5.5	15.6	15.8
Real Estate
Net Investment Income [Line Items]
Investment income (loss)	2.5	2.2	7.0	6.7
Investment expenses	2.0	1.8	6.3	6.1
Company-Owned Life Insurance
Net Investment Income [Line Items]
Investment income (loss)	10.8	9.7	31.5	25.7
Universal Life-type Policyholder Account Balances
Net Investment Income [Line Items]
Investment income (loss)	3.1	2.2	8.4	7.3
Other Investment Expenses
Net Investment Income [Line Items]
Investment expenses	4.2	5.0	13.9	14.3
Interest Expense, Other Short-Term Borrowings	$ 4.7	$ 4.8	$ 14.3	$ 15.4

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