INVESTMENTS - Unrealized Losses (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	$ 26.2	$ 36.1
Less than 12 months, unrealized loss	0.0	(0.1)
12 months or more, fair value	49.7	72.3
12 months or more, unrealized loss	(1.5)	(3.1)
Total, fair value	75.9	108.4
Total, unrealized loss	(1.5)	(3.2)
U.S. Treasury securities and agencies
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	2.2	1.7
Less than 12 months, unrealized loss	0.0	0.0
12 months or more, fair value	2.5	2.4
12 months or more, unrealized loss	0.0	(0.1)
Total, fair value	4.7	4.1
Total, unrealized loss	0.0	(0.1)
Municipal securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	0.1	3.1
Less than 12 months, unrealized loss	0.0	0.0
12 months or more, fair value	11.2	15.1
12 months or more, unrealized loss	(0.4)	(0.8)
Total, fair value	11.3	18.2
Total, unrealized loss	(0.4)	(0.8)
Corporate debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	14.2	13.4
Less than 12 months, unrealized loss	0.0	0.0
12 months or more, fair value	23.2	35.1
12 months or more, unrealized loss	(0.6)	(1.3)
Total, fair value	37.4	48.5
Total, unrealized loss	(0.6)	(1.3)
Residential mortgage-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	1.6	4.1
Less than 12 months, unrealized loss	0.0	0.0
12 months or more, fair value	1.7	1.9
12 months or more, unrealized loss	(0.1)	(0.2)
Total, fair value	3.3	6.0
Total, unrealized loss	(0.1)	(0.2)
Commercial mortgage-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	6.2	9.1
Less than 12 months, unrealized loss	0.0	(0.1)
12 months or more, fair value	8.8	13.4
12 months or more, unrealized loss	(0.4)	(0.6)
Total, fair value	15.0	22.5
Total, unrealized loss	(0.4)	(0.7)
Other debt obligations
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	0.9	4.4
Less than 12 months, unrealized loss	0.0	0.0
12 months or more, fair value	2.3	4.4
12 months or more, unrealized loss	0.0	(0.1)
Total, fair value	3.2	8.8
Total, unrealized loss	0.0	(0.1)
Total fixed maturities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	25.2	35.8
Less than 12 months, unrealized loss	0.0	(0.1)
12 months or more, fair value	49.7	72.3
12 months or more, unrealized loss	(1.5)	(3.1)
Total, fair value	74.9	108.1
Total, unrealized loss	(1.5)	(3.2)
Short-term investments
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	1.0	0.3
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	1.0	0.3
Total, unrealized loss	$ 0.0	$ 0.0