INVESTMENTS - Unrealized Losses (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	$ 65.8	$ 143.0
Less than 12 months, unrealized loss	(0.4)	(1.6)
12 months or more, fair value	35.0	43.9
12 months or more, unrealized loss	(1.1)	(1.5)
Total, fair value	100.8	186.9
Total, unrealized loss	(1.5)	(3.1)
U.S. Treasury securities and agencies
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	7.0	26.4
Less than 12 months, unrealized loss	(0.1)	(0.3)
12 months or more, fair value	1.8	1.8
12 months or more, unrealized loss	0.0	0.0
Total, fair value	8.8	28.2
Total, unrealized loss	(0.1)	(0.3)
Municipal securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	2.6	7.1
Less than 12 months, unrealized loss	0.0	(0.1)
12 months or more, fair value	8.9	10.4
12 months or more, unrealized loss	(0.3)	(0.4)
Total, fair value	11.5	17.5
Total, unrealized loss	(0.3)	(0.5)
Corporate debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	26.8	49.5
Less than 12 months, unrealized loss	(0.2)	(0.6)
12 months or more, fair value	15.6	21.1
12 months or more, unrealized loss	(0.4)	(0.6)
Total, fair value	42.4	70.6
Total, unrealized loss	(0.6)	(1.2)
Asset-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	21.5	53.6
Less than 12 months, unrealized loss	(0.1)	(0.6)
12 months or more, fair value	8.7	10.6
12 months or more, unrealized loss	(0.4)	(0.5)
Total, fair value	30.2	64.2
Total, unrealized loss	(0.5)	(1.1)
Total fixed maturities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	57.9	136.6
Less than 12 months, unrealized loss	(0.4)	(1.6)
12 months or more, fair value	35.0	43.9
12 months or more, unrealized loss	(1.1)	(1.5)
Total, fair value	92.9	180.5
Total, unrealized loss	(1.5)	(3.1)
Short-term investments
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	7.9	6.4
Less than 12 months, unrealized loss	0.0	0.0
12 months or more, fair value	0.0	0.0
12 months or more, unrealized loss	0.0	0.0
Total, fair value	7.9	6.4
Total, unrealized loss	$ 0.0	$ 0.0