Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Net Investment Income [Line Items]
Investment income (loss)	$ 117.9	$ 113.7	$ 324.9	$ 335.0
Investment expenses	6.8	6.7	20.4	19.9
Net Investment Income	111.1	107.0	304.5	315.1
Fixed Maturities
Net Investment Income [Line Items]
Investment income (loss)	79.0	81.2	239.0	242.3
Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	0.9	1.2	4.6	3.4
Total Alternative Investments
Net Investment Income [Line Items]
Investment income (loss)	10.0	9.7	2.8	22.5
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income (loss)	0.9	4.3	(15.9)	8.2
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	9.1	5.4	18.7	14.3
Short-term Investments
Net Investment Income [Line Items]
Investment income (loss)	8.4	6.0	23.0	11.9
Loans to Policyholders
Net Investment Income [Line Items]
Investment income (loss)	5.5	5.1	15.8	15.6
Real Estate
Net Investment Income [Line Items]
Investment income (loss)	2.2	2.3	6.7	6.6
Investment expenses	1.8	1.7	6.1	6.0
Universal Life-type Policyholder Account Balances
Net Investment Income [Line Items]
Investment income (loss)	2.2	1.8	7.3	10.1
Company-Owned Life Insurance
Net Investment Income [Line Items]
Investment income (loss)	9.7	6.4	25.7	22.6
Other Investment Expenses
Net Investment Income [Line Items]
Investment expenses	5.0	5.0	14.3	13.9
Interest Expense, Other Short-Term Borrowings	$ 4.8	$ 5.3	$ 15.4	$ 17.5