Income from Investments - Net Investment Income (Details) - USD ($)	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Net Investment Income [Line Items]
Investment income	$ 434,200,000	$ 447,100,000	$ 450,600,000
Investment expenses	26,700,000	27,400,000	28,000,000.0
Net investment income	407,500,000	419,700,000	422,600,000
Interest on Fixed Income Securities
Net Investment Income [Line Items]
Investment income	315,300,000	323,300,000	290,000,000.0
Dividends on Equity Securities Excluding Alternative Investments
Net Investment Income [Line Items]
Investment income	5,400,000	4,400,000	6,300,000
Total Alternative Investments
Net Investment Income [Line Items]
Investment income	6,300,000	29,500,000	73,400,000
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income		10,500,000	31,300,000
Investment expenses	18,200,000
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income	24,500,000	19,000,000.0	42,100,000
Short-term Investments
Net Investment Income [Line Items]
Investment income	33,500,000	18,000,000.0	3,700,000
Loans to Policyholders
Net Investment Income [Line Items]
Investment income	21,000,000.0	20,900,000	21,500,000
Real Estate
Net Investment Income [Line Items]
Investment income	8,800,000	8,900,000	10,100,000
Investment expenses	8,700,000	8,800,000	7,900,000
Other
Net Investment Income [Line Items]
Investment income	8,200,000	12,900,000	7,700,000
Investment expenses	18,000,000.0	18,600,000	20,100,000
Cash Surrender Value
Net Investment Income [Line Items]
Investment income	35,700,000	29,200,000	37,900,000
Universal Life-type Policyholder Account Balances
Net Investment Income [Line Items]
Interest Expense, Other Short-Term Borrowings	$ 20,300,000	$ 22,700,000	$ 10,100,000