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INVESTMENTS - SUMMARY OF INVESTMENTS WITH UNREALIZED LOSSES BY INVESTMENT CATEGORY (Details) - USD ($)
$ in Millions
Mar. 31, 2025
Dec. 31, 2024
Fair value    
Less than 12 months $ 2,296.5 $ 2,982.0
12 months or greater 7,351.3 8,488.3
Total 9,647.8 11,470.3
Unrealized losses    
Less than 12 months (35.9) (70.7)
12 months or greater (987.0) (1,358.4)
Total (1,022.9) (1,429.1)
Corporate securities    
Fair value    
Less than 12 months 731.5 1,200.7
12 months or greater 3,133.2 4,035.6
Total 3,864.7 5,236.3
Unrealized losses    
Less than 12 months (20.9) (35.5)
12 months or greater (481.9) (740.7)
Total (502.8) (776.2)
United States Treasury securities and obligations of United States government corporations and agencies    
Fair value    
Less than 12 months 44.2 44.7
12 months or greater 147.4 141.5
Total 191.6 186.2
Unrealized losses    
Less than 12 months (2.9) (3.8)
12 months or greater (20.0) (24.8)
Total (22.9) (28.6)
States and political subdivisions    
Fair value    
Less than 12 months 289.8 831.9
12 months or greater 803.2 896.1
Total 1,093.0 1,728.0
Unrealized losses    
Less than 12 months (4.6) (20.5)
12 months or greater (154.3) (212.1)
Total (158.9) (232.6)
Foreign governments    
Fair value    
Less than 12 months 6.2 17.4
12 months or greater 2.6 10.0
Total 8.8 27.4
Unrealized losses    
Less than 12 months (0.1) (1.0)
12 months or greater (0.1) (1.1)
Total (0.2) (2.1)
Asset-backed securities    
Fair value    
Less than 12 months 105.7 124.8
12 months or greater 788.9 807.9
Total 894.6 932.7
Unrealized losses    
Less than 12 months (0.8) (1.3)
12 months or greater (56.5) (64.3)
Total (57.3) (65.6)
Agency residential mortgage-backed securities    
Fair value    
Less than 12 months 197.0 297.1
12 months or greater 3.0 3.1
Total 200.0 300.2
Unrealized losses    
Less than 12 months (1.5) (5.3)
12 months or greater (0.1) (0.2)
Total (1.6) (5.5)
Non-agency residential mortgage-backed securities    
Fair value    
Less than 12 months 85.5 128.0
12 months or greater 863.9 884.6
Total 949.4 1,012.6
Unrealized losses    
Less than 12 months (0.7) (1.4)
12 months or greater (111.1) (129.4)
Total (111.8) (130.8)
Collateralized loan obligations    
Fair value    
Less than 12 months 644.0 162.9
12 months or greater 56.0 66.8
Total 700.0 229.7
Unrealized losses    
Less than 12 months (3.7) (0.7)
12 months or greater (2.8) (3.3)
Total (6.5) (4.0)
Commercial mortgage-backed securities    
Fair value    
Less than 12 months 192.6 174.5
12 months or greater 1,553.1 1,642.7
Total 1,745.7 1,817.2
Unrealized losses    
Less than 12 months (0.7) (1.2)
12 months or greater (160.2) (182.5)
Total $ (160.9) $ (183.7)