INVESTMENTS - Carrying Amounts of Investments (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 343.0	$ 331.6
Gross Unrealized Gains	2.0	3.5
Gross Unrealized Losses	(3.4)	(2.2)
Fair Value	341.6	332.9
Interest receivable	2.4	2.5
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6.9	6.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.1)	(0.1)
Fair Value	6.9	6.2
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15.4	13.6
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	(0.2)	(0.1)
Fair Value	15.2	13.5
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	29.6	30.1
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	(0.3)	(0.2)
Fair Value	29.4	30.1
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	142.0	131.5
Gross Unrealized Gains	0.9	1.6
Gross Unrealized Losses	(1.5)	(0.9)
Fair Value	141.4	132.2
Mortgage and other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	149.1	150.1
Gross Unrealized Gains	1.0	1.6
Gross Unrealized Losses	(1.3)	(0.9)
Fair Value	$ 148.7	$ 150.9