INVESTMENTS - Unrealized Losses (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 35.9	$ 29.6
Less than 12 Months, Unrealized Loss	(0.3)	(0.1)
12 Months or More, Fair Value	0.3	0.0
12 Months or More, Unrealized Loss	0.0	0.0
Total, Fair Value	36.2	29.6
Total, Unrealized Loss	(0.3)	(0.1)
U.S. Treasury securities and agencies
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	14.0	15.7
Less than 12 Months, Unrealized Loss	(0.1)	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	14.0	15.7
Total, Unrealized Loss	(0.1)	0.0
Municipal securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	6.4	2.3
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	6.4	2.3
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	8.1	2.9
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	8.1	2.9
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	4.3	3.7
Less than 12 Months, Unrealized Loss	(0.1)	0.0
12 Months or More, Fair Value	0.3	0.0
12 Months or More, Unrealized Loss	0.0	0.0
Total, Fair Value	4.6	3.7
Total, Unrealized Loss	(0.1)	0.0
Commercial mortgage backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	3.1	4.9
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	3.1	4.9
Total, Unrealized Loss	$ 0.0	0.0
Other debt obligations
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

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