Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Net Investment Income
Total net investment income	$ 108.5	$ 105.2	$ 208.9	$ 203.1
Investment Portfolio
Net Investment Income
Investment expenses	(2.6)	(2.5)	(6.2)	(5.1)
Total net investment income	82.5	79.4	157.2	152.4
Deposit Asset on Reinsurance
Net Investment Income
Total net investment income	26.0	25.8	51.7	50.7
Fixed maturity securities \| Investment Portfolio
Net Investment Income
Net investment income	65.2	62.0	132.9	120.6
Equity securities \| Investment Portfolio
Net Investment Income
Net investment income	0.7	4.0	3.4	5.3
Limited partnership interests \| Investment Portfolio
Net Investment Income
Net investment income	15.7	13.2	20.1	26.2
Short-term and other investments \| Investment Portfolio
Net Investment Income
Net investment income	$ 3.5	$ 2.7	$ 7.0	$ 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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