Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Net Investment Income [Line Items]
Investment income (loss)	$ 108.5	$ 108.3
Investment expenses	7.3	7.9
Net Investment Income	101.2	100.4
Fixed Maturities
Net Investment Income [Line Items]
Investment income (loss)	76.4	79.8
Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	0.8	2.8
Total Alternative Investments
Net Investment Income [Line Items]
Investment income (loss)	3.0	1.3
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income (loss)	(0.7)	(2.1)
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	3.7	3.4
Short-term Investments
Net Investment Income [Line Items]
Investment income (loss)	8.6	7.3
Loans to Policyholders
Net Investment Income [Line Items]
Investment income (loss)	5.3	5.2
Real Estate
Net Investment Income [Line Items]
Investment income (loss)	2.2	2.3
Investment expenses	2.1	2.2
Company-Owned Life Insurance
Net Investment Income [Line Items]
Investment income (loss)	10.2	7.1
Universal Life-type Policyholder Account Balances
Net Investment Income [Line Items]
Investment income (loss)	2.0	2.5
Other Investment Expenses
Net Investment Income [Line Items]
Investment expenses	5.2	5.7
Interest Expense, Other Short-Term Borrowings	$ 4.8	$ 5.2