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INVESTMENTS - Schedule of Gross Unrealized Losses and Fair Values for Investments in Unrealized Loss Position (Details) - USD ($)
$ in Millions
Sep. 30, 2023
Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than 12 months $ 21.4 $ 353.9
Gross unrealized losses, less than 12 months (0.4) (8.1)
Fair value, 12 months or greater 953.4 1,111.4
Gross unrealized losses, 12 months or greater (38.3) (60.0)
Fair value, total 974.8 1,465.3
Gross unrealized losses, total (38.7) (68.1)
U.S. government and agency securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than 12 months 2.6 61.6
Gross unrealized losses, less than 12 months (0.1) (1.5)
Fair value, 12 months or greater 76.7 69.5
Gross unrealized losses, 12 months or greater (4.2) (4.6)
Fair value, total 79.3 131.1
Gross unrealized losses, total (4.3) (6.1)
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than 12 months 18.8 103.3
Gross unrealized losses, less than 12 months (0.3) (1.3)
Fair value, 12 months or greater 200.1 254.6
Gross unrealized losses, 12 months or greater (7.5) (12.7)
Fair value, total 218.9 357.9
Gross unrealized losses, total (7.8) (14.0)
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than 12 months 0.0 189.0
Gross unrealized losses, less than 12 months 0.0 (5.3)
Fair value, 12 months or greater 674.0 784.8
Gross unrealized losses, 12 months or greater (26.4) (42.5)
Fair value, total 674.0 973.8
Gross unrealized losses, total (26.4) (47.8)
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 2.5
Gross unrealized losses, 12 months or greater (0.2) (0.2)
Fair value, total 2.6 2.5
Gross unrealized losses, total $ (0.2) $ (0.2)