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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
Jun. 30, 2024
Mar. 31, 2024
Dec. 31, 2023
Jun. 30, 2023
Mar. 31, 2023
Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]            
Amortized cost $ 25,106.1          
Allowance for credit losses (39.8) $ (39.0) $ (42.9) $ (66.1) $ (59.1) $ (56.0)
Estimated fair value 22,617.9   21,506.2      
Corporate securities            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 13,450.7   13,186.9      
Gross unrealized gains 43.4   74.7      
Gross unrealized losses (1,667.4)   (1,382.4)      
Allowance for credit losses (38.6) (37.7) (41.7) (64.4) (57.2) (54.4)
Estimated fair value 11,788.1   11,837.5      
Certificates of deposit            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 470.2          
Gross unrealized gains 3.5          
Gross unrealized losses 0.0          
Allowance for credit losses 0.0          
Estimated fair value 473.7          
United States Treasury securities and obligations of United States government corporations and agencies            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 278.6   207.6      
Gross unrealized gains 0.0   0.1      
Gross unrealized losses (22.1)   (13.3)      
Allowance for credit losses 0.0   0.0      
Estimated fair value 256.5   194.4      
States and political subdivisions            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 3,192.5   2,896.8      
Gross unrealized gains 18.6   31.3      
Gross unrealized losses (399.7)   (360.7)      
Allowance for credit losses (0.6) (0.6) (0.7) (1.0) (0.8) (0.9)
Estimated fair value 2,810.8   2,566.7      
Foreign governments            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 101.1   92.7      
Gross unrealized gains 0.4   1.2      
Gross unrealized losses (13.3)   (10.4)      
Allowance for credit losses (0.5) (0.6) (0.4) (0.5) (0.5) (0.4)
Estimated fair value 87.7   83.1      
Asset-backed securities            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 1,552.2   1,476.2      
Gross unrealized gains 4.5   4.1      
Gross unrealized losses (87.9)   (107.8)      
Allowance for credit losses (0.1) $ (0.1) (0.1) $ (0.2) $ (0.6) $ (0.3)
Estimated fair value 1,468.7   1,372.4      
Agency residential mortgage-backed securities            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 787.3   639.0      
Gross unrealized gains 6.1   9.5      
Gross unrealized losses (4.0)   (0.5)      
Allowance for credit losses 0.0   0.0      
Estimated fair value 789.4   648.0      
Non-agency residential mortgage-backed securities            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 1,667.2   1,670.1      
Gross unrealized gains 30.4   35.8      
Gross unrealized losses (144.8)   (152.7)      
Allowance for credit losses 0.0   0.0      
Estimated fair value 1,552.8   1,553.2      
Collateralized loan obligations            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 1,227.6   1,042.5      
Gross unrealized gains 5.1   3.3      
Gross unrealized losses (7.8)   (13.0)      
Allowance for credit losses 0.0   0.0      
Estimated fair value 1,224.9   1,032.8      
Commercial mortgage-backed securities            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 2,378.7   2,487.4      
Gross unrealized gains 1.2   0.7      
Gross unrealized losses (214.6)   (270.0)      
Allowance for credit losses 0.0   0.0      
Estimated fair value 2,165.3   2,218.1      
Total fixed maturities, available for sale            
Debt Securities, Available-for-sale [Line Items]            
Amortized cost 25,106.1   23,699.2      
Gross unrealized gains 113.2   160.7      
Gross unrealized losses (2,561.6)   (2,310.8)      
Allowance for credit losses (39.8)   (42.9)      
Estimated fair value $ 22,617.9   $ 21,506.2