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INVESTMENTS - Carrying Amounts of Investments (Details) - USD ($)
$ in Millions
Jun. 30, 2024
Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 351.1 $ 331.6
Gross Unrealized Gains 1.3 3.5
Gross Unrealized Losses (4.3) (2.2)
Fair Value 348.1 332.9
Short-term investments    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 10.6 6.3
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.1) (0.1)
Fair Value 10.5 6.2
U.S. Treasury    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 17.1 13.6
Gross Unrealized Gains 0.0 0.1
Gross Unrealized Losses (0.3) (0.1)
Fair Value 16.9 13.5
Municipal    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 29.5 30.1
Gross Unrealized Gains 0.0 0.2
Gross Unrealized Losses (0.5) (0.2)
Fair Value 29.1 30.1
Corporate    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 149.2 131.5
Gross Unrealized Gains 0.6 1.6
Gross Unrealized Losses (1.8) (0.9)
Fair Value 148.0 132.2
Mortgage and other asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 144.6 150.1
Gross Unrealized Gains 0.7 1.6
Gross Unrealized Losses (1.6) (0.9)
Fair Value $ 143.7 $ 150.9