Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Net Investment Income
Total net investment income	$ 115.9	$ 105.4
Investment Portfolio
Net Investment Income
Investment expenses	(2.8)	(2.6)
Total net investment income	91.5	79.9
Deposit Asset on Reinsurance
Net Investment Income
Total net investment income	24.4	25.5
Fixed maturity securities \| Investment Portfolio
Net Investment Income
Net investment income	71.6	70.4
Equity securities \| Investment Portfolio
Net Investment Income
Net investment income	1.0	1.2
Limited partnership interests \| Investment Portfolio
Net Investment Income
Net investment income	16.3	6.2
Short-term and other investments \| Investment Portfolio
Net Investment Income
Net investment income	$ 5.4	$ 4.7

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