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INVESTMENTS - SCHEDULE OF AMORTIZED COST, GROSS UNREALIZED GAINS AND LOSSES, ESTIMATED FAIR VALUE, AND ALLOWANCE FOR CREDIT LOSSES (Details) - USD ($)
$ in Millions
Sep. 30, 2025
Jun. 30, 2025
Dec. 31, 2024
Sep. 30, 2024
Jun. 30, 2024
Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]            
Amortized cost $ 25,219.3          
Allowance for credit losses (33.0) $ (39.1) $ (37.1) $ (25.9) $ (39.8) $ (42.9)
Estimated fair value 23,405.3   22,730.1      
Corporate securities            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 14,226.1   13,672.1      
Gross unrealized gains 158.2   60.2      
Gross unrealized losses (1,352.0)   (1,660.4)      
Allowance for credit losses (27.5) $ (33.1) (31.1) $ (24.5) $ (38.6) $ (41.7)
Estimated fair value 13,004.8   12,040.8      
Certificates of deposit            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 0.0   470.0      
Gross unrealized gains 0.0   18.3      
Gross unrealized losses 0.0   0.0      
Allowance for credit losses 0.0   0.0      
Estimated fair value 0.0   488.3      
United States Treasury securities and obligations of United States government corporations and agencies            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 206.2   214.8      
Gross unrealized gains 0.0   0.0      
Gross unrealized losses (26.8)   (28.6)      
Allowance for credit losses 0.0   0.0      
Estimated fair value 179.4   186.2      
States and political subdivisions            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 3,346.9   3,261.9      
Gross unrealized gains 27.3   12.2      
Gross unrealized losses (375.7)   (436.4)      
Allowance for credit losses (2.8)   (3.4)      
Estimated fair value 2,995.7   2,834.3      
Foreign governments            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 121.9   107.2      
Gross unrealized gains 1.1   0.1      
Gross unrealized losses (10.7)   (15.3)      
Allowance for credit losses (0.6)   (0.9)      
Estimated fair value 111.7   91.1      
Asset-backed securities            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 1,547.1   1,574.6      
Gross unrealized gains 11.9   8.3      
Gross unrealized losses (43.2)   (66.4)      
Allowance for credit losses (0.1)   (0.1)      
Estimated fair value 1,515.7   1,516.4      
Agency residential mortgage-backed securities            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 896.0   819.8      
Gross unrealized gains 11.9   5.3      
Gross unrealized losses (0.5)   (5.5)      
Allowance for credit losses 0.0   0.0      
Estimated fair value 907.4   819.6      
Non-agency residential mortgage-backed securities            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 1,569.7   1,636.3      
Gross unrealized gains 38.6   33.6      
Gross unrealized losses (96.9)   (130.8)      
Allowance for credit losses 0.0   0.0      
Estimated fair value 1,511.4   1,539.1      
Collateralized loan obligations            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 1,127.4   1,015.2      
Gross unrealized gains 3.2   5.6      
Gross unrealized losses (4.5)   (4.0)      
Allowance for credit losses 0.0   0.0      
Estimated fair value 1,126.1   1,016.8      
Commercial mortgage-backed securities            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 2,178.0   2,379.1      
Gross unrealized gains 5.9   3.7      
Gross unrealized losses (128.8)   (183.7)      
Allowance for credit losses (2.0)   (1.6)      
Estimated fair value 2,053.1   2,197.5      
Total fixed maturities, available for sale            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 25,219.3   25,151.0      
Gross unrealized gains 258.1   147.3      
Gross unrealized losses (2,039.1)   (2,531.1)      
Allowance for credit losses (33.0)   (37.1)      
Estimated fair value $ 23,405.3   $ 22,730.1