v3.26.1
Investments Fixed Income Securities by Type (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Schedule of Fixed Income Securities [Table] [Line Items]    
Fixed income securities (amortized cost) $ 12,116.5  
Fixed income securities (estimated fair value) 12,161.2 $ 12,709.8
Debt Securities, Available-for-sale, Allowance for Credit Loss 1.6 1.6
U.S. & Canadian Governments [Member]    
Schedule of Fixed Income Securities [Table] [Line Items]    
Fixed income securities (amortized cost) 1,695.8 1,795.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 2.5 13.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 33.2 21.1
Fixed income securities (estimated fair value) 1,665.0 1,787.4
Corporate [Member]    
Schedule of Fixed Income Securities [Table] [Line Items]    
Fixed income securities (amortized cost) 10,420.7 10,683.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 120.7 255.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 45.2 16.9
Fixed income securities (estimated fair value) 10,496.2 10,922.3
Fixed Income Securities [Member]    
Schedule of Fixed Income Securities [Table] [Line Items]    
Fixed income securities (amortized cost) 12,116.5 12,478.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 123.2 269.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 78.5 38.1
Fixed income securities (estimated fair value) $ 12,161.2 $ 12,709.8