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INVESTMENTS - SUMMARY OF INVESTMENTS WITH UNREALIZED LOSSES BY INVESTMENT CATEGORY (Details) - USD ($)
$ in Millions
Sep. 30, 2025
Dec. 31, 2024
Fair value    
Less than 12 months $ 1,019.0 $ 2,982.1
12 months or greater 6,920.8 8,482.0
Total 7,939.8 11,464.1
Unrealized losses    
Less than 12 months (10.9) (70.6)
12 months or greater (873.1) (1,357.8)
Total (884.0) (1,428.4)
Corporate securities    
Fair value    
Less than 12 months 198.2 1,200.8
12 months or greater 3,153.4 4,029.2
Total 3,351.6 5,230.0
Unrealized losses    
Less than 12 months (3.4) (35.5)
12 months or greater (449.7) (740.3)
Total (453.1) (775.8)
United States Treasury securities and obligations of United States government corporations and agencies    
Fair value    
Less than 12 months 18.3 44.7
12 months or greater 154.2 141.5
Total 172.5 186.2
Unrealized losses    
Less than 12 months (1.7) (3.8)
12 months or greater (25.1) (24.8)
Total (26.8) (28.6)
States and political subdivisions    
Fair value    
Less than 12 months 203.9 831.9
12 months or greater 861.7 896.1
Total 1,065.6 1,728.0
Unrealized losses    
Less than 12 months (2.0) (20.5)
12 months or greater (128.1) (212.1)
Total (130.1) (232.6)
Foreign governments    
Fair value    
Less than 12 months 0.0 17.4
12 months or greater 20.8 10.0
Total 20.8 27.4
Unrealized losses    
Less than 12 months 0.0 (1.0)
12 months or greater (0.9) (1.0)
Total (0.9) (2.0)
Asset-backed securities    
Fair value    
Less than 12 months 37.9 124.8
12 months or greater 621.9 807.9
Total 659.8 932.7
Unrealized losses    
Less than 12 months (0.3) (1.2)
12 months or greater (42.1) (64.3)
Total (42.4) (65.5)
Agency residential mortgage-backed securities    
Fair value    
Less than 12 months 21.3 297.1
12 months or greater 45.1 3.1
Total 66.4 300.2
Unrealized losses    
Less than 12 months (0.1) (5.3)
12 months or greater (0.4) (0.2)
Total (0.5) (5.5)
Non-agency residential mortgage-backed securities    
Fair value    
Less than 12 months 84.2 128.0
12 months or greater 762.5 884.6
Total 846.7 1,012.6
Unrealized losses    
Less than 12 months (0.4) (1.4)
12 months or greater (96.5) (129.4)
Total (96.9) (130.8)
Collateralized loan obligations    
Fair value    
Less than 12 months 284.3 162.9
12 months or greater 66.8 66.9
Total 351.1 229.8
Unrealized losses    
Less than 12 months (2.4) (0.7)
12 months or greater (2.1) (3.2)
Total (4.5) (3.9)
Commercial mortgage-backed securities    
Fair value    
Less than 12 months 170.9 174.5
12 months or greater 1,234.4 1,642.7
Total 1,405.3 1,817.2
Unrealized losses    
Less than 12 months (0.6) (1.2)
12 months or greater (128.2) (182.5)
Total $ (128.8) $ (183.7)