INVESTMENTS - Schedule of Gross Realized Gains and Losses (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
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)
Total Fair Value	153.6	152.6
Total Gross Unrealized Losses	(2.2)	(2.5)
U.S Treasury
Debt Securities, Available-for-sale [Line Items]
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
Total Fair Value	8.5	9.2
Total Gross Unrealized Losses	(0.1)	(0.2)
Municipal
Debt Securities, Available-for-sale [Line Items]
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
Total Fair Value	16.8	19.0
Total Gross Unrealized Losses	(0.2)	(0.4)
Corporate
Debt Securities, Available-for-sale [Line Items]
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)
Total Fair Value	59.4	71.4
Total Gross Unrealized Losses	(0.9)	(0.4)
Mortgage and other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
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)
Total Fair Value	62.9	52.9
Total Gross Unrealized Losses	$ (0.9)	$ (1.5)