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SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details)
$ in Millions
Dec. 31, 2025
USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost $ 10,404.8 [1]
Fair Value 10,374.4
Amount at which shown in the balance sheet 10,382.7 [2]
U.S. Treasury and Government Agencies [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 2,500.7 [1]
Fair Value 2,410.8
Amount at which shown in the balance sheet 2,410.8 [2]
States, municipalities and political subdivisions [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 910.8 [1]
Fair Value 847.7
Amount at which shown in the balance sheet 847.7 [2]
Foreign governments [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 2.5 [1]
Fair Value 2.7
Amount at which shown in the balance sheet 2.7 [2]
Public Utilities Bonds [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 353.5 [1]
Fair Value 355.5
Amount at which shown in the balance sheet 355.5 [2]
All other corporate bonds [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 5,916.6 [1]
Fair Value 5,918.0
Amount at which shown in the balance sheet 5,918.0 [2]
Fixed Maturities [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 9,684.1 [1]
Fair Value 9,534.7
Amount at which shown in the balance sheet 9,534.7 [2]
Public utilities [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 10.2 [1]
Fair Value 26.3
Amount at which shown in the balance sheet 26.3 [2]
Banks, trust and insurance companies [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 1.6 [1]
Fair Value 3.0
Amount at which shown in the balance sheet 3.0 [2]
Industrial, miscellaneous and all other [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 53.5 [1]
Fair Value 136.5
Amount at which shown in the balance sheet 136.5 [2]
Nonredeemable preferred stock [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 12.4 [1]
Fair Value 19.0
Amount at which shown in the balance sheet 19.0 [2]
Equity Securities Investment Summary [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 77.7 [1]
Fair Value 184.8
Amount at which shown in the balance sheet 184.8 [2]
Mortgage Loans on Real Estate [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 255.1 [1]
Fair Value 238.7
Amount at which shown in the balance sheet 247.0 [2]
Other long-term investments [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 386.1 [1]
Fair Value 414.4
Amount at which shown in the balance sheet 414.4 [2]
Short-Term Investments [Member]  
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]  
Cost 1.8 [1]
Fair Value 1.8
Amount at which shown in the balance sheet $ 1.8 [2]
[1] Original cost of equity securities and, as to fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums and accretion of discounts.
[2] Mortgage loans on real estate are shown on the balance sheet at unpaid principal balances adjusted for deferred fees or expenses, net of an allowance for credit losses.