INVESTMENTS - Schedule of Unrealized Losses on Debt Securities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	$ 35.2	$ 353.9
Gross unrealized losses, less than 12 months	(1.9)	(8.1)
Fair value, 12 months or greater	843.7	1,111.4
Gross unrealized losses, 12 months or greater	(25.6)	(60.0)
Total fair value	878.9	1,465.3
Total gross unrealized losses	(27.5)	(68.1)
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	0.0	61.6
Gross unrealized losses, less than 12 months	0.0	(1.5)
Fair value, 12 months or greater	67.1	69.5
Gross unrealized losses, 12 months or greater	(2.8)	(4.6)
Total fair value	67.1	131.1
Total gross unrealized losses	(2.8)	(6.1)
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	10.2	103.3
Gross unrealized losses, less than 12 months	(1.8)	(1.3)
Fair value, 12 months or greater	172.7	254.6
Gross unrealized losses, 12 months or greater	(6.0)	(12.7)
Total fair value	182.9	357.9
Total gross unrealized losses	(7.8)	(14.0)
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	25.0	189.0
Gross unrealized losses, less than 12 months	(0.1)	(5.3)
Fair value, 12 months or greater	601.3	784.8
Gross unrealized losses, 12 months or greater	(16.6)	(42.5)
Total fair value	626.3	973.8
Total gross unrealized losses	(16.7)	(47.8)
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	0.0	0.0
Gross unrealized losses, less than 12 months	0.0	0.0
Fair value, 12 months or greater	2.6	2.5
Gross unrealized losses, 12 months or greater	(0.2)	(0.2)
Total fair value	2.6	2.5
Total gross unrealized losses	$ (0.2)	$ (0.2)