INVESTMENTS - Unrealized Losses (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 29.6	$ 52.1
Less than 12 Months, Unrealized Loss	(0.1)	(0.2)
12 Months or More, Fair Value	0.0	0.0
12 Months or More, Unrealized Loss	0.0	0.0
Total, Fair Value	29.6	52.1
Total, Unrealized Loss	(0.1)	(0.2)
U.S. Treasury securities and agencies
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	15.7
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	15.7
Total, Unrealized Loss	0.0
Municipal securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	2.3	5.5
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	2.3	5.5
Total, Unrealized Loss	0.0	0.0
Corporate debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	2.9	12.8
Less than 12 Months, Unrealized Loss	(0.1)	(0.1)
12 Months or More, Fair Value	0.0	0.0
12 Months or More, Unrealized Loss	0.0	0.0
Total, Fair Value	2.9	12.8
Total, Unrealized Loss	(0.1)	(0.1)
Residential mortgage-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	3.7	1.9
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	3.7	1.9
Total, Unrealized Loss	0.0	0.0
Commercial mortgage backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	4.9	24.5
Less than 12 Months, Unrealized Loss	0.0	(0.1)
12 Months or More, Fair Value	0.0	0.0
12 Months or More, Unrealized Loss	0.0	0.0
Total, Fair Value	4.9	24.5
Total, Unrealized Loss	0.0	(0.1)
Other debt obligations
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	0.1	7.4
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	0.1	7.4
Total, Unrealized Loss	$ 0.0	$ 0.0