Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Net Investment Income [Line Items]
Investment income (loss)	$ 108.3	$ 108.1
Investment expenses	7.9	6.3
Net Investment Income	100.4	101.8
Fixed Maturities
Net Investment Income [Line Items]
Investment income (loss)	79.8	81.5
Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	2.8	1.0
Total Alternative Investments
Net Investment Income [Line Items]
Investment income (loss)	1.3	3.7
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income (loss)	(2.1)	1.1
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	3.4	2.6
Short-term Investments
Net Investment Income [Line Items]
Investment income (loss)	7.3	2.3
Loans to Policyholders
Net Investment Income [Line Items]
Investment income (loss)	5.2	5.4
Real Estate
Net Investment Income [Line Items]
Investment income (loss)	2.3	2.4
Investment expenses	2.2	2.1
Other Investment Expenses1,2
Net Investment Income [Line Items]
Investment expenses	5.7	4.2
Interest Expense, Other Short-Term Borrowings	5.2	4.7
Universal Life-type Policyholder Account Balances
Net Investment Income [Line Items]
Investment income (loss)	2.5	3.0
Cash Surrender Value
Net Investment Income [Line Items]
Investment income (loss)	$ 7.1	$ 8.8