INVESTMENTS - Carrying Amounts of Investments (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 331.6	$ 244.2
Gross Unrealized Gains	3.5	0.5
Gross Unrealized Losses	(2.2)	(4.4)
Fair Value	332.9	240.4
Interest receivable	2.5	1.3
Less than 12 months, fair value	51.1	145.7
Less than 12 months, gross unrealized losses	(0.7)	(2.0)
12 months or longer, fair value	102.5	6.8
12 months or longer, gross unrealized losses	(1.6)	(0.5)
Fair Value	153.6	152.6
Unrealized Losses	(2.2)	(2.5)
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6.3	5.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.1)	0.0
Fair Value	6.2	5.4
Less than 12 months, fair value	0.0
Less than 12 months, gross unrealized losses	0.0
12 months or longer, fair value	6.0
12 months or longer, gross unrealized losses	(0.1)
Fair Value	6.0
Unrealized Losses	(0.1)
U.S Treasury
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13.6	11.8
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(0.1)	(0.2)
Fair Value	13.5	11.6
Less than 12 months, fair value	3.4	9.2
Less than 12 months, gross unrealized losses	(0.1)	(0.2)
12 months or longer, fair value	5.0	0.0
12 months or longer, gross unrealized losses	(0.1)	0.0
Fair Value	8.5	9.2
Unrealized Losses	(0.1)	(0.2)
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	30.1	22.8
Gross Unrealized Gains	0.2	0.0
Gross Unrealized Losses	(0.2)	(0.4)
Fair Value	30.1	22.4
Less than 12 months, fair value	6.4	19.0
Less than 12 months, gross unrealized losses	(0.1)	(0.4)
12 months or longer, fair value	10.4	0.0
12 months or longer, gross unrealized losses	(0.1)	0.0
Fair Value	16.8	19.0
Unrealized Losses	(0.2)	(0.4)
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	131.5	81.8
Gross Unrealized Gains	1.6	0.2
Gross Unrealized Losses	(0.9)	(2.3)
Fair Value	132.2	79.7
Less than 12 months, fair value	11.4	66.2
Less than 12 months, gross unrealized losses	(0.1)	(0.1)
12 months or longer, fair value	48.0	5.2
12 months or longer, gross unrealized losses	(0.8)	(0.3)
Fair Value	59.4	71.4
Unrealized Losses	(0.9)	(0.4)
Mortgage and other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	150.1	73.8
Gross Unrealized Gains	1.6	0.3
Gross Unrealized Losses	(0.9)	(1.4)
Fair Value	150.9	72.7
Less than 12 months, fair value	29.8	51.4
Less than 12 months, gross unrealized losses	(0.4)	(1.3)
12 months or longer, fair value	33.1	1.5
12 months or longer, gross unrealized losses	(0.5)	(0.2)
Fair Value	62.9	52.9
Unrealized Losses	(0.9)	(1.5)
Total debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	331.6	195.5
Gross Unrealized Gains	3.5	0.5
Gross Unrealized Losses	(2.2)	(4.4)
Fair Value	$ 332.9	191.7
Common Stock
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		48.7
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Fair Value		$ 48.7