Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Net Investment Income [Line Items]
Investment income (loss)	$ 102.6	$ 98.7	$ 211.1	$ 207.0
Investment expenses	6.7	5.7	14.0	13.6
Net Investment Income	95.9	93.0	197.1	193.4
Fixed Maturities
Net Investment Income [Line Items]
Investment income (loss)	76.9	80.2	153.3	160.0
Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	0.7	0.9	1.5	3.7
Total Alternative Investments
Net Investment Income [Line Items]
Investment income (loss)	(2.2)	(8.5)	0.8	(7.2)
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income (loss)	(5.3)	(14.7)	(6.0)	(16.8)
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	3.1	6.2	6.8	9.6
Short-term Investments
Net Investment Income [Line Items]
Investment income (loss)	6.0	7.3	14.6	14.6
Loans to Policyholders
Net Investment Income [Line Items]
Investment income (loss)	5.1	5.1	10.4	10.3
Real Estate
Net Investment Income [Line Items]
Investment income (loss)	2.3	2.2	4.5	4.5
Investment expenses	2.2	2.1	4.3	4.3
Company-Owned Life Insurance
Net Investment Income [Line Items]
Investment income (loss)	10.5	8.9	20.7	16.0
Universal Life-type Policyholder Account Balances
Net Investment Income [Line Items]
Investment income (loss)	3.3	2.6	5.3	5.1
Other Investment Expenses
Net Investment Income [Line Items]
Investment expenses	4.5	3.6	9.7	9.3
Interest Expense, Other Short-Term Borrowings	$ 4.8	$ 5.4	$ 9.6	$ 10.6