Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Net Investment Income [Line Items]
Total net investment income	$ 110.7	$ 115.9
Investment Portfolio
Net Investment Income [Line Items]
Investment expenses	(3.1)	(2.8)
Total net investment income	87.1	91.5
Deposit Asset on Reinsurance
Net Investment Income [Line Items]
Total net investment income	23.6	24.4
Fixed maturity securities \| Investment Portfolio
Net Investment Income [Line Items]
Net investment income	71.2	71.6
Equity securities \| Investment Portfolio
Net Investment Income [Line Items]
Net investment income	0.5	1.0
Limited partnership interests \| Investment Portfolio
Net Investment Income [Line Items]
Net investment income	13.2	16.3
Short-term and other investments \| Investment Portfolio
Net Investment Income [Line Items]
Net investment income	$ 5.3	$ 5.4

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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