v3.25.4
Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance, Beginning of Year $ 209.0 $ 192.3
Total Gains (Losses):    
Included in Consolidated Statements of Income (Loss) $ 2.3 2.8
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Net Realized Investment Gains (Losses)  
Included in Other Comprehensive Income $ (5.9) 0.2
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration] Changes in Unrealized Gains (Losses) on Investment Securities with:  
Purchases $ 443.5 133.4
Sales (190.8) (105.7)
Transfers into Level 3 16.7 10.6
Transfers out of Level 3 (131.2) (24.6)
Balance, End of Year 343.6 209.0
Bonds and Notes    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance, Beginning of Year 194.2 177.1
Total Gains (Losses):    
Included in Consolidated Statements of Income (Loss) 2.2 0.6
Included in Other Comprehensive Income (6.3) 0.8
Purchases 279.5 124.2
Sales (175.2) (104.0)
Transfers into Level 3 5.0 7.1
Transfers out of Level 3 (6.4) (11.6)
Balance, End of Year 293.0 194.2
States and Political Subdivisions    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance, Beginning of Year 1.8 0.1
Total Gains (Losses):    
Included in Consolidated Statements of Income (Loss) 0.0 0.0
Included in Other Comprehensive Income (0.3) (0.5)
Purchases 0.0 0.0
Sales 0.0 0.0
Transfers into Level 3 1.7 3.5
Transfers out of Level 3 (1.5) (1.3)
Balance, End of Year 1.7 1.8
Redeemable Preferred Stocks    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance, Beginning of Year 4.2 7.1
Total Gains (Losses):    
Included in Consolidated Statements of Income (Loss) 0.2 0.0
Included in Other Comprehensive Income 0.0 0.1
Purchases 0.0 1.9
Sales 0.0 0.0
Transfers into Level 3 0.0 0.0
Transfers out of Level 3 0.0 (4.9)
Balance, End of Year 4.4 4.2
Collateralized Loan Obligations    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance, Beginning of Year 0.0 0.0
Total Gains (Losses):    
Included in Consolidated Statements of Income (Loss) 0.0 0.0
Included in Other Comprehensive Income 0.0 0.0
Purchases 137.9 6.8
Sales (10.0) 0.0
Transfers into Level 3 0.0 0.0
Transfers out of Level 3 (115.5) (6.8)
Balance, End of Year 12.4 0.0
Other Mortgage- and Asset-backed    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance, Beginning of Year 5.0 5.2
Total Gains (Losses):    
Included in Consolidated Statements of Income (Loss) (0.1) 0.0
Included in Other Comprehensive Income 0.1 (0.2)
Purchases 20.8 0.0
Sales (0.1) 0.0
Transfers into Level 3 10.0 0.0
Transfers out of Level 3 (7.8) 0.0
Balance, End of Year 27.9 5.0
Equity Securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance, Beginning of Year 3.8 2.8
Total Gains (Losses):    
Included in Consolidated Statements of Income (Loss) 0.0 2.2
Included in Other Comprehensive Income 0.6 0.0
Purchases 0.3 0.5
Sales (0.5) (1.7)
Transfers into Level 3 0.0 0.0
Transfers out of Level 3 0.0 0.0
Balance, End of Year 4.2 3.8
Foreign Governments    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance, Beginning of Year 0.0  
Total Gains (Losses):    
Included in Consolidated Statements of Income (Loss) 0.0  
Included in Other Comprehensive Income 0.0  
Purchases 5.0  
Sales (5.0)  
Transfers into Level 3 0.0  
Transfers out of Level 3 0.0  
Balance, End of Year $ 0.0 $ 0.0