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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
Mar. 31, 2026
Dec. 31, 2025
Mar. 31, 2025
Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]        
Amortized cost $ 26,081.4      
Allowance for credit losses (43.9) $ (36.0) $ (38.9) $ (37.1)
Estimated fair value 23,881.1 23,886.8    
Corporate securities        
Debt Securities, Available-for-sale [Line Items]        
Amortized cost 14,558.6 14,568.8    
Gross unrealized gains 39.6 137.1    
Gross unrealized losses (1,578.6) (1,413.2)    
Allowance for credit losses (37.8) (30.2) $ (33.1) $ (31.1)
Estimated fair value 12,981.8 13,262.5    
United States Treasury securities and obligations of United States government corporations and agencies        
Debt Securities, Available-for-sale [Line Items]        
Amortized cost 208.9 207.2    
Gross unrealized gains 0.0 0.1    
Gross unrealized losses (32.5) (30.2)    
Allowance for credit losses 0.0 0.0    
Estimated fair value 176.4 177.1    
States and political subdivisions        
Debt Securities, Available-for-sale [Line Items]        
Amortized cost 3,250.8 3,311.4    
Gross unrealized gains 21.4 25.1    
Gross unrealized losses (390.2) (378.9)    
Allowance for credit losses (2.8) (2.9)    
Estimated fair value 2,879.2 2,954.7    
Foreign governments        
Debt Securities, Available-for-sale [Line Items]        
Amortized cost 134.8 130.9    
Gross unrealized gains 0.4 0.9    
Gross unrealized losses (13.4) (10.7)    
Allowance for credit losses (1.0) (0.6)    
Estimated fair value 120.8 120.5    
Asset-backed securities        
Debt Securities, Available-for-sale [Line Items]        
Amortized cost 1,844.1 1,780.8    
Gross unrealized gains 7.8 14.0    
Gross unrealized losses (51.4) (47.4)    
Allowance for credit losses (0.1) (0.1)    
Estimated fair value 1,800.4 1,747.3    
Agency residential mortgage-backed securities        
Debt Securities, Available-for-sale [Line Items]        
Amortized cost 810.6 838.3    
Gross unrealized gains 9.5 11.4    
Gross unrealized losses (0.9) (0.2)    
Allowance for credit losses 0.0 0.0    
Estimated fair value 819.2 849.5    
Non-agency residential mortgage-backed securities        
Debt Securities, Available-for-sale [Line Items]        
Amortized cost 1,556.2 1,639.1    
Gross unrealized gains 32.7 37.5    
Gross unrealized losses (85.5) (90.9)    
Allowance for credit losses 0.0 0.0    
Estimated fair value 1,503.4 1,585.7    
Collateralized loan obligations        
Debt Securities, Available-for-sale [Line Items]        
Amortized cost 1,466.5 1,142.4    
Gross unrealized gains 1.3 2.8    
Gross unrealized losses (4.2) (2.7)    
Allowance for credit losses 0.0 0.0    
Estimated fair value 1,463.6 1,142.5    
Commercial mortgage-backed securities        
Debt Securities, Available-for-sale [Line Items]        
Amortized cost 2,250.9 2,157.5    
Gross unrealized gains 4.1 6.4    
Gross unrealized losses (116.5) (114.7)    
Allowance for credit losses (2.2) (2.2)    
Estimated fair value 2,136.3 2,047.0    
Total fixed maturities, available for sale        
Debt Securities, Available-for-sale [Line Items]        
Amortized cost 26,081.4 25,776.4    
Gross unrealized gains 116.8 235.3    
Gross unrealized losses (2,273.2) (2,088.9)    
Allowance for credit losses (43.9) (36.0)    
Estimated fair value $ 23,881.1 $ 23,886.8