Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Net Investment Income
Total net investment income	$ 118.9	$ 97.6	$ 327.8	$ 300.7
Investment Portfolio
Net Investment Income
Investment expenses	(3.3)	(2.7)	(9.5)	(7.8)
Total net investment income	91.8	70.9	249.0	223.3
Deposit Asset on Reinsurance
Net Investment Income
Total net investment income	27.1	26.7	78.8	77.4
Fixed maturity securities \| Investment Portfolio
Net Investment Income
Net investment income	68.7	63.9	201.6	184.5
Equity securities \| Investment Portfolio
Net Investment Income
Net investment income	1.3	1.8	4.7	7.1
Limited partnership interests \| Investment Portfolio
Net Investment Income
Net investment income	20.5	5.1	40.6	31.3
Short-term and other investments \| Investment Portfolio
Net Investment Income
Net investment income	$ 4.6	$ 2.8	$ 11.6	$ 8.2

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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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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