Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
Net Investment Income [Line Items]
Investment income	$ 112.8	$ 108.9
Investment expenses	11.0	8.9
Net Investment Income	101.8	100.0
Fixed Maturities
Net Investment Income [Line Items]
Investment income	86.2	68.7
Equity Securities
Net Investment Income [Line Items]
Investment income	1.0	1.5
Total Alternative Investments
Net Investment Income [Line Items]
Investment income	3.7	20.9
Equity Method Limited Liability Investments
Net Investment Income [Line Items]
Investment income	1.1	13.3
Limited Liability Investments Included in Equity Securities
Net Investment Income [Line Items]
Investment income	2.6	7.6
Short-term Investments
Net Investment Income [Line Items]
Investment income	2.3	0.1
Loans to Policyholders
Net Investment Income [Line Items]
Investment income	5.4	5.5
Real Estate
Net Investment Income [Line Items]
Investment income	2.4	2.2
Investment expenses	2.1	2.5
Other Investment Expenses
Net Investment Income [Line Items]
Investment expenses	8.9	6.4
Universal Life-type Policyholder Account Balances
Net Investment Income [Line Items]
Investment income	3.0	1.7
Cash Surrender Value
Net Investment Income [Line Items]
Investment income	$ 8.8	$ 8.3

Amount, before 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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