INVESTMENTS - Schedule of Gross Realized Gains and Losses (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Gain (Loss) on Securities [Line Items]
Less than 12 Months, Fair Value	$ 95.4	$ 51.1
Less than 12 Months, Gross Unrealized Losses	(1.2)	(0.7)
12 Months or Longer, Fair Value	103.4	102.5
12 Months or Longer, Gross Unrealized Losses	(2.2)	(1.6)
Total Fair Value	198.7	153.6
Total Gross Unrealized Losses	(3.4)	(2.2)
Short-term investments
Gain (Loss) on Securities [Line Items]
Less than 12 Months, Fair Value	0.2	0.0
Less than 12 Months, Gross Unrealized Losses	0.0	0.0
12 Months or Longer, Fair Value	4.5	6.0
12 Months or Longer, Gross Unrealized Losses	(0.1)	(0.1)
Total Fair Value	4.7	6.0
Total Gross Unrealized Losses	(0.1)	(0.1)
U.S. Treasury
Gain (Loss) on Securities [Line Items]
Less than 12 Months, Fair Value	5.4	3.4
Less than 12 Months, Gross Unrealized Losses	(0.1)	(0.1)
12 Months or Longer, Fair Value	4.8	5.0
12 Months or Longer, Gross Unrealized Losses	(0.1)	(0.1)
Total Fair Value	10.2	8.5
Total Gross Unrealized Losses	(0.2)	(0.1)
Municipal
Gain (Loss) on Securities [Line Items]
Less than 12 Months, Fair Value	8.7	6.4
Less than 12 Months, Gross Unrealized Losses	(0.1)	(0.1)
12 Months or Longer, Fair Value	13.0	10.4
12 Months or Longer, Gross Unrealized Losses	(0.2)	(0.1)
Total Fair Value	21.7	16.8
Total Gross Unrealized Losses	(0.3)	(0.2)
Corporate
Gain (Loss) on Securities [Line Items]
Less than 12 Months, Fair Value	31.1	11.4
Less than 12 Months, Gross Unrealized Losses	(0.3)	(0.1)
12 Months or Longer, Fair Value	52.1	48.0
12 Months or Longer, Gross Unrealized Losses	(1.2)	(0.8)
Total Fair Value	83.2	59.4
Total Gross Unrealized Losses	(1.5)	(0.9)
Mortgage and other asset-backed securities
Gain (Loss) on Securities [Line Items]
Less than 12 Months, Fair Value	50.0	29.8
Less than 12 Months, Gross Unrealized Losses	(0.7)	(0.4)
12 Months or Longer, Fair Value	29.0	33.1
12 Months or Longer, Gross Unrealized Losses	(0.6)	(0.5)
Total Fair Value	79.0	62.9
Total Gross Unrealized Losses	$ (1.3)	$ (0.9)