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Investments - Net Investment Income (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Net Investment Income [Line Items]      
Total net investment income $ 444.8 $ 400.9 [1] $ 422.5 [1]
Investment Portfolio      
Net Investment Income [Line Items]      
Investment expenses (12.1) (10.5) (10.1)
Total net investment income 339.9 297.4 321.4
Deposit Asset On Reinsurance      
Net Investment Income [Line Items]      
Total net investment income 104.9 103.5 101.1
Fixed maturity securities | Investment Portfolio      
Net Investment Income [Line Items]      
Investment income 269.2 247.2 235.6
Equity securities | Investment Portfolio      
Net Investment Income [Line Items]      
Investment income 6.5 9.0 5.3
Limited partnership interests | Investment Portfolio      
Net Investment Income [Line Items]      
Investment income 59.1 40.5 79.0
Short-term and other investments | Investment Portfolio      
Net Investment Income [Line Items]      
Investment income $ 17.2 $ 11.2 $ 11.6
[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements