INVESTMENTS - Unrealized Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	$ 91.1	$ 39.7
Less than 12 months, unrealized loss	(3.2)	(0.4)
12 months or more, fair value	30.3	16.1
12 months or more, unrealized loss	(2.7)	(0.4)
Total, fair value	121.4	55.8
Total, unrealized loss	(5.9)	(0.8)
Realized gains on investments	1.2	2.5	$ 0.5
Realized losses on investments	(0.7)	(0.1)	(0.2)
Net realized gains on investments	0.5	2.4	$ 0.3
U.S. Treasury securities and agencies
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	6.9	7.5
Less than 12 months, unrealized loss	(0.1)	(0.1)
12 months or more, fair value	4.1	14.0
12 months or more, unrealized loss	(0.2)	(0.3)
Total, fair value	11.0	21.5
Total, unrealized loss	(0.3)	(0.4)
Municipal securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	11.5	8.9
Less than 12 months, unrealized loss	(0.5)	(0.1)
12 months or more, fair value	8.2	0.0
12 months or more, unrealized loss	(0.7)	0.0
Total, fair value	19.7	8.9
Total, unrealized loss	(1.2)	(0.1)
Corporate debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	45.3	12.7
Less than 12 months, unrealized loss	(1.6)	(0.1)
12 months or more, fair value	11.5	1.6
12 months or more, unrealized loss	(1.1)	(0.1)
Total, fair value	56.8	14.3
Total, unrealized loss	(2.7)	(0.2)
Residential mortgage-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	2.2	1.9
Less than 12 months, unrealized loss	0.0	0.0
12 months or more, fair value	1.9	0.5
12 months or more, unrealized loss	(0.3)	0.0
Total, fair value	4.1	2.4
Total, unrealized loss	(0.3)	0.0
Commercial mortgage-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	18.3	8.7
Less than 12 months, unrealized loss	(0.8)	(0.1)
12 months or more, fair value	4.6	0.0
12 months or more, unrealized loss	(0.4)	0.0
Total, fair value	22.9	8.7
Total, unrealized loss	(1.2)	$ (0.1)
Other debt obligations
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	6.8
Less than 12 months, unrealized loss	(0.2)
12 months or more, fair value	0.0
12 months or more, unrealized loss	0.0
Total, fair value	6.8
Total, unrealized loss	(0.2)
Total fixed maturities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	91.0
Less than 12 months, unrealized loss	(3.2)
12 months or more, fair value	30.3
12 months or more, unrealized loss	(2.7)
Total, fair value	121.3
Total, unrealized loss	(5.9)
Short-term investments
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	0.1
Less than 12 months, unrealized loss	0.0
12 months or more, fair value	0.0
12 months or more, unrealized loss	0.0
Total, fair value	0.1
Total, unrealized loss	$ 0.0