Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Net Investment Income [Line Items]
Investment income (loss)	$ 113.7	$ 102.6	$ 227.7	$ 211.1
Investment expenses	8.3	6.7	15.2	14.0
Net Investment Income	105.4	95.9	212.5	197.1
Fixed Maturities
Net Investment Income [Line Items]
Investment income (loss)	80.9	76.9	160.3	153.3
Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	2.1	0.7	2.8	1.5
Total Alternative Investments
Net Investment Income [Line Items]
Investment income (loss)	4.7	(2.2)	12.4	0.8
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income (loss)	1.2	(5.3)	1.8	(6.0)
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	3.5	3.1	10.6	6.8
Short-term Investments
Net Investment Income [Line Items]
Investment income (loss)	2.7	6.0	5.8	14.6
Loans to Policyholders
Net Investment Income [Line Items]
Investment income (loss)	5.2	5.1	10.5	10.4
Real Estate
Net Investment Income [Line Items]
Investment income (loss)	2.7	2.3	5.0	4.5
Investment expenses	2.7	2.2	4.5	4.3
Company-Owned Life Insurance
Net Investment Income [Line Items]
Investment income (loss)	12.1	10.5	23.6	20.7
Other
Net Investment Income [Line Items]
Investment income (loss)	3.3	3.3	7.3	5.3
Other Investment Expenses
Net Investment Income [Line Items]
Investment expenses	5.6	4.5	10.7	9.7
Interest Expense, Other Short-Term Borrowings	$ 3.5	$ 4.8	$ 7.4	$ 9.6

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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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