Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Net Investment Income
Total net investment income	$ 108.4	$ 108.5	$ 213.8	$ 208.9
Investment Portfolio
Net Investment Income
Investment expenses	(3.2)	(2.6)	(5.8)	(6.2)
Total net investment income	82.7	82.5	162.6	157.2
Deposit Asset on Reinsurance
Net Investment Income
Total net investment income	25.7	26.0	51.2	51.7
Fixed maturity securities \| Investment Portfolio
Net Investment Income
Net investment income	72.0	65.2	142.4	132.9
Equity securities \| Investment Portfolio
Net Investment Income
Net investment income	1.4	0.7	2.6	3.4
Limited partnership interests \| Investment Portfolio
Net Investment Income
Net investment income	8.2	15.7	14.4	20.1
Short-term and other investments \| Investment Portfolio
Net Investment Income
Net investment income	$ 4.3	$ 3.5	$ 9.0	$ 7.0

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