INVESTMENTS - Amortized Cost and Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 272.7	$ 169.3
Allowance for Expected Credit Losses	0.0	0.0
Gross Unrealized Gains	3.6	0.7
Gross Unrealized Losses	(1.5)	(3.2)
Fair Value	274.8	166.8
U.S. Treasury securities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	33.7	14.6
Allowance for Expected Credit Losses	0.0	0.0
Gross Unrealized Gains	0.6	0.1
Gross Unrealized Losses	0.0	(0.1)
Fair Value	34.3	14.6
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	23.4	24.8
Allowance for Expected Credit Losses	0.0	0.0
Gross Unrealized Gains	0.3	0.1
Gross Unrealized Losses	(0.4)	(0.8)
Fair Value	23.3	24.1
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	104.2	66.3
Allowance for Expected Credit Losses	0.0	0.0
Gross Unrealized Gains	1.4	0.2
Gross Unrealized Losses	(0.6)	(1.3)
Fair Value	105.0	65.2
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20.7	12.0
Allowance for Expected Credit Losses	0.0	0.0
Gross Unrealized Gains	0.3	0.1
Gross Unrealized Losses	(0.1)	(0.2)
Fair Value	20.9	11.9
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	52.0	30.4
Allowance for Expected Credit Losses	0.0	0.0
Gross Unrealized Gains	0.7	0.1
Gross Unrealized Losses	(0.4)	(0.7)
Fair Value	52.3	29.8
Other debt obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	30.1	20.3
Allowance for Expected Credit Losses	0.0	0.0
Gross Unrealized Gains	0.3	0.1
Gross Unrealized Losses	0.0	(0.1)
Fair Value	30.4	20.3
Total fixed maturities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	264.1	168.4
Allowance for Expected Credit Losses	0.0	0.0
Gross Unrealized Gains	3.6	0.7
Gross Unrealized Losses	(1.5)	(3.2)
Fair Value	266.2	165.9
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8.6	0.9
Allowance for Expected Credit Losses	0.0	0.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 8.6	$ 0.9