Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Net Investment Income
Total net investment income	$ 113.0	$ 118.9	$ 326.8	$ 327.8
Investment Portfolio
Net Investment Income
Investment expenses	(4.0)	(3.3)	(9.8)	(9.5)
Total net investment income	87.2	91.8	249.8	249.0
Deposit Asset on Reinsurance
Net Investment Income
Total net investment income	25.8	27.1	77.0	78.8
Fixed maturity securities \| Investment Portfolio
Net Investment Income
Net investment income	69.4	68.7	211.8	201.6
Equity securities \| Investment Portfolio
Net Investment Income
Net investment income	1.3	1.3	3.9	4.7
Limited partnership interests \| Investment Portfolio
Net Investment Income
Net investment income	14.5	20.5	28.9	40.6
Short-term and other investments \| Investment Portfolio
Net Investment Income
Net investment income	$ 6.0	$ 4.6	$ 15.0	$ 11.6

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