Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Net Investment Income [Line Items]
Total net investment income	$ 119.6	$ 113.0	$ 346.3	$ 326.8
Investment Portfolio
Net Investment Income [Line Items]
Investment expenses	(3.2)	(4.0)	(9.1)	(9.8)
Total net investment income	96.4	87.2	273.6	249.8
Deposit Asset on Reinsurance
Net Investment Income [Line Items]
Total net investment income	23.2	25.8	72.7	77.0
Fixed maturity securities \| Investment Portfolio
Net Investment Income [Line Items]
Net investment income	71.5	69.4	202.5	211.8
Equity securities \| Investment Portfolio
Net Investment Income [Line Items]
Net investment income	0.9	1.3	2.9	3.9
Limited partnership interests \| Investment Portfolio
Net Investment Income [Line Items]
Net investment income	21.4	14.5	60.9	28.9
Short-term and other investments \| Investment Portfolio
Net Investment Income [Line Items]
Net investment income	$ 5.8	$ 6.0	$ 16.4	$ 15.0

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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