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INVESTMENTS - Schedule of Gross Unrealized Losses and Fair Values for Investments in Unrealized Loss Position (Details) - USD ($)
$ in Millions
Mar. 31, 2024
Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than 12 months $ 12.2 $ 35.2
Gross unrealized losses, less than 12 months (0.1) (1.9)
Fair value, 12 months or greater 622.1 843.7
Gross unrealized losses, 12 months or greater (17.9) (25.6)
Fair value, total 634.3 878.9
Gross unrealized losses, total (18.0) (27.5)
U.S. government and agency securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than 12 months 0.0 0.0
Gross unrealized losses, less than 12 months 0.0 0.0
Fair value, 12 months or greater 58.5 67.1
Gross unrealized losses, 12 months or greater (2.4) (2.8)
Fair value, total 58.5 67.1
Gross unrealized losses, total (2.4) (2.8)
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than 12 months 12.2 10.2
Gross unrealized losses, less than 12 months (0.1) (1.8)
Fair value, 12 months or greater 130.7 172.7
Gross unrealized losses, 12 months or greater (3.7) (6.0)
Fair value, total 142.9 182.9
Gross unrealized losses, total (3.8) (7.8)
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than 12 months 0.0 25.0
Gross unrealized losses, less than 12 months 0.0 (0.1)
Fair value, 12 months or greater 430.3 601.3
Gross unrealized losses, 12 months or greater (11.7) (16.6)
Fair value, total 430.3 626.3
Gross unrealized losses, total (11.7) (16.7)
Municipal securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than 12 months 0.0 0.0
Gross unrealized losses, less than 12 months 0.0 0.0
Fair value, 12 months or greater 2.6 2.6
Gross unrealized losses, 12 months or greater (0.1) (0.2)
Fair value, total 2.6 2.6
Gross unrealized losses, total $ (0.1) $ (0.2)