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Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) - USD ($)
$ in Millions
Dec. 31, 2022
Dec. 31, 2021
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Investment Owned, at Cost [1] $ 15,176.6  
Investments 15,859.9 $ 16,576.3
Short-term Investments [Member]    
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Cost [1] 860.8  
Amount at which shown in balance sheet 860.8  
Other investments [Member]    
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Cost [1] 31.2  
Amount at which shown in balance sheet 31.2  
Fixed maturity securities [Member] | United States Government and government agencies and authorities [Member]    
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Cost [1] 2,099.5  
Fair Value 1,995.2  
Amount at which shown in balance sheet 1,995.2  
Fixed maturity securities [Member] | States, municipalities and political subdivisions [Member]    
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Cost [1] 882.5  
Fair Value 869.7  
Amount at which shown in balance sheet 869.7  
Fixed maturity securities [Member] | Foreign government [Member]    
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Cost [1] 200.5  
Fair Value 189.9  
Amount at which shown in balance sheet 189.9  
Fixed maturity securities [Member] | Corporate, industrial and all other [Member]    
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Cost [1] 9,153.7  
Fair Value 8,691.7  
Amount at which shown in balance sheet 8,691.7  
Fixed maturity securities [Member] | Fixed Maturities [Member]    
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Cost [1] 12,336.3  
Fair Value 11,746.7  
Amount at which shown in balance sheet 11,746.7  
Equity Securities [Member] | Non-redeemable preferred stock [Member]    
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Cost [1] 0.6  
Fair Value 1.0  
Amount at which shown in balance sheet 1.0  
Equity Securities [Member] | Banks, trust and insurance companies [Member]    
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Cost [1] 84.3  
Fair Value 165.2  
Amount at which shown in balance sheet 165.2  
Equity Securities [Member] | Industrial, miscellaneous, and all others [Member]    
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Cost [1] 1,863.1  
Fair Value 3,054.6  
Amount at which shown in balance sheet 3,054.6  
Equity Securities [Member] | Equity Securities [Member]    
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Cost [1] 1,948.1  
Fair Value 3,220.9  
Amount at which shown in balance sheet 3,220.9  
Fixed Maturity and Short-Term Investments    
Summary of Investments, Other than Investments in Related Parties, Reportable Data    
Cost 13,197.2  
Amount at which shown in balance sheet $ 12,607.6  
[1] (1)    Represents original cost of equity securities, and as to fixed incomes, original cost reduced by repayments and adjusted for amortization of premium or accrual of discount.