Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Net Investment Income [Line Items]
Investment income (loss)	$ 98.7	$ 113.2	$ 207.0	$ 221.3
Investment expenses	5.7	6.9	13.6	13.2
Net Investment Income	93.0	106.3	193.4	208.1
Fixed Maturities
Net Investment Income [Line Items]
Investment income (loss)	80.2	79.6	160.0	161.1
Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	0.9	1.2	3.7	2.2
Total Alternative Investments
Net Investment Income [Line Items]
Investment income (loss)	(8.5)	9.1	(7.2)	12.8
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income (loss)	(14.7)	2.8	(16.8)	3.9
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income (loss)	6.2	6.3	9.6	8.9
Short-term Investments
Net Investment Income [Line Items]
Investment income (loss)	7.3	3.6	14.6	5.9
Loans to Policyholders
Net Investment Income [Line Items]
Investment income (loss)	5.1	5.1	10.3	10.5
Real Estate
Net Investment Income [Line Items]
Investment income (loss)	2.2	1.9	4.5	4.3
Investment expenses	2.1	2.2	4.3	4.3
Other Investment Expenses
Net Investment Income [Line Items]
Investment expenses	3.6	4.7	9.3	8.9
Interest Expense, Other Short-Term Borrowings	5.4	7.5	10.6	12.2
Cash Surrender Value
Net Investment Income [Line Items]
Investment income (loss)	8.9	7.4	16.0	16.2
Universal Life-type Policyholder Account Balances
Net Investment Income [Line Items]
Investment income (loss)	$ 2.6	$ 5.3	$ 5.1	$ 8.3

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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