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INVESTMENTS - SUMMARY OF INVESTMENTS WITH UNREALIZED LOSSES BY INVESTMENT CATEGORY (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Fair value    
Less than 12 months $ 4,728.1 $ 1,864.0
12 months or greater 5,045.5 5,964.9
Total 9,773.6 7,828.9
Unrealized losses    
Less than 12 months (99.3) (58.5)
12 months or greater (827.8) (836.7)
Total (927.1) (895.2)
Corporate securities    
Fair value    
Less than 12 months 2,103.4 734.9
12 months or greater 1,866.3 2,645.8
Total 3,969.7 3,380.7
Unrealized losses    
Less than 12 months (75.5) (44.3)
12 months or greater (411.6) (426.7)
Total (487.1) (471.0)
United States Treasury securities and obligations of United States government corporations and agencies    
Fair value    
Less than 12 months 18.9 0.4
12 months or greater 153.3 153.9
Total 172.2 154.3
Unrealized losses    
Less than 12 months (0.2) 0.0
12 months or greater (32.3) (30.2)
Total (32.5) (30.2)
States and political subdivisions    
Fair value    
Less than 12 months 261.4 275.6
12 months or greater 846.8 806.0
Total 1,108.2 1,081.6
Unrealized losses    
Less than 12 months (9.9) (8.5)
12 months or greater (136.5) (127.6)
Total (146.4) (136.1)
Foreign governments    
Fair value    
Less than 12 months 8.1 3.1
12 months or greater 17.4 24.3
Total 25.5 27.4
Unrealized losses    
Less than 12 months (0.2) 0.0
12 months or greater (3.2) (2.7)
Total (3.4) (2.7)
Asset-backed securities    
Fair value    
Less than 12 months 618.0 187.8
12 months or greater 441.5 487.7
Total 1,059.5 675.5
Unrealized losses    
Less than 12 months (5.1) (1.8)
12 months or greater (45.5) (44.9)
Total (50.6) (46.7)
Agency residential mortgage-backed securities    
Fair value    
Less than 12 months 100.6 59.8
12 months or greater 13.7 13.8
Total 114.3 73.6
Unrealized losses    
Less than 12 months (0.6) 0.0
12 months or greater (0.3) (0.2)
Total (0.9) (0.2)
Non-agency residential mortgage-backed securities    
Fair value    
Less than 12 months 239.5 131.4
12 months or greater 624.8 703.3
Total 864.3 834.7
Unrealized losses    
Less than 12 months (1.7) (0.9)
12 months or greater (83.9) (90.0)
Total (85.6) (90.9)
Collateralized loan obligations    
Fair value    
Less than 12 months 772.4 227.3
12 months or greater 72.9 62.9
Total 845.3 290.2
Unrealized losses    
Less than 12 months (2.4) (0.9)
12 months or greater (1.8) (1.8)
Total (4.2) (2.7)
Commercial mortgage-backed securities    
Fair value    
Less than 12 months 605.8 243.7
12 months or greater 1,008.8 1,067.2
Total 1,614.6 1,310.9
Unrealized losses    
Less than 12 months (3.7) (2.1)
12 months or greater (112.7) (112.6)
Total $ (116.4) $ (114.7)