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INVESTMENTS - SUMMARY OF INVESTMENTS WITH UNREALIZED LOSSES BY INVESTMENT CATEGORY (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Fair value    
Less than 12 months $ 4,900.2 $ 1,864.0
12 months or greater 4,801.9 5,964.9
Total 9,702.1 7,828.9
Unrealized losses    
Less than 12 months (98.6) (58.5)
12 months or greater (797.5) (836.7)
Total (896.1) (895.2)
Corporate securities    
Fair value    
Less than 12 months 2,486.7 734.9
12 months or greater 1,816.2 2,645.8
Total 4,302.9 3,380.7
Unrealized losses    
Less than 12 months (74.7) (44.3)
12 months or greater (387.3) (426.7)
Total (462.0) (471.0)
United States Treasury securities and obligations of United States government corporations and agencies    
Fair value    
Less than 12 months 22.6 0.4
12 months or greater 155.0 153.9
Total 177.6 154.3
Unrealized losses    
Less than 12 months (0.2) 0.0
12 months or greater (32.4) (30.2)
Total (32.6) (30.2)
States and political subdivisions    
Fair value    
Less than 12 months 293.7 275.6
12 months or greater 879.4 806.0
Total 1,173.1 1,081.6
Unrealized losses    
Less than 12 months (10.7) (8.5)
12 months or greater (147.6) (127.6)
Total (158.3) (136.1)
Foreign governments    
Fair value    
Less than 12 months 7.2 3.1
12 months or greater 10.1 24.3
Total 17.3 27.4
Unrealized losses    
Less than 12 months (0.1) 0.0
12 months or greater (2.1) (2.7)
Total (2.2) (2.7)
Asset-backed securities    
Fair value    
Less than 12 months 789.2 187.8
12 months or greater 409.9 487.7
Total 1,199.1 675.5
Unrealized losses    
Less than 12 months (5.5) (1.8)
12 months or greater (42.5) (44.9)
Total (48.0) (46.7)
Agency residential mortgage-backed securities    
Fair value    
Less than 12 months 147.9 59.8
12 months or greater 6.6 13.8
Total 154.5 73.6
Unrealized losses    
Less than 12 months (0.8) 0.0
12 months or greater (0.1) (0.2)
Total (0.9) (0.2)
Non-agency residential mortgage-backed securities    
Fair value    
Less than 12 months 205.9 131.4
12 months or greater 563.9 703.3
Total 769.8 834.7
Unrealized losses    
Less than 12 months (2.2) (0.9)
12 months or greater (76.2) (90.0)
Total (78.4) (90.9)
Collateralized loan obligations    
Fair value    
Less than 12 months 481.0 227.3
12 months or greater 46.9 62.9
Total 527.9 290.2
Unrealized losses    
Less than 12 months (1.1) (0.9)
12 months or greater (1.5) (1.8)
Total (2.6) (2.7)
Commercial mortgage-backed securities    
Fair value    
Less than 12 months 466.0 243.7
12 months or greater 913.9 1,067.2
Total 1,379.9 1,310.9
Unrealized losses    
Less than 12 months (3.3) (2.1)
12 months or greater (107.8) (112.6)
Total $ (111.1) $ (114.7)