INVESTMENTS - SUMMARY OF CHANGES IN THE ALLOWANCE FOR CURRENT EXPECTED CREDIT LOSSES (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance at the beginning of the period	$ 7.6	$ 2.2
Additions for securities for which credit losses were not previously recorded	14.4	1.8
Additions for purchased securities with deteriorated credit	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	15.0	1.5
Reduction for securities sold during the period	(0.4)	(0.2)
Reduction for securities for which the Company made the decision to sell where an allowance was previously recorded	0.0	0.0
Write-offs	0.0	0.0
Recoveries of previously written-off amount	0.0	0.0
Allowance at the end of the period	36.6	5.3
Corporate securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance at the beginning of the period	7.4	1.9
Additions for securities for which credit losses were not previously recorded	14.0	1.7
Additions for purchased securities with deteriorated credit	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	14.6	1.5
Reduction for securities sold during the period	(0.4)	(0.2)
Reduction for securities for which the Company made the decision to sell where an allowance was previously recorded	0.0	0.0
Write-offs	0.0	0.0
Recoveries of previously written-off amount	0.0	0.0
Allowance at the end of the period	35.6	4.9
States and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance at the beginning of the period	0.0	0.3
Additions for securities for which credit losses were not previously recorded	0.3	0.1
Additions for purchased securities with deteriorated credit	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	0.5	0.0
Reduction for securities sold during the period	0.0	0.0
Reduction for securities for which the Company made the decision to sell where an allowance was previously recorded	0.0	0.0
Write-offs	0.0	0.0
Recoveries of previously written-off amount	0.0	0.0
Allowance at the end of the period	0.8	$ 0.4
Foreign governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance at the beginning of the period	0.2
Additions for securities for which credit losses were not previously recorded	0.1
Additions for purchased securities with deteriorated credit	0.0
Additions (reductions) for securities where an allowance was previously recorded	(0.2)
Reduction for securities sold during the period	0.0
Reduction for securities for which the Company made the decision to sell where an allowance was previously recorded	0.0
Write-offs	0.0
Recoveries of previously written-off amount	0.0
Allowance at the end of the period	0.1
Asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance at the beginning of the period	0.0
Additions for securities for which credit losses were not previously recorded	0.0
Additions for purchased securities with deteriorated credit	0.0
Additions (reductions) for securities where an allowance was previously recorded	0.1
Reduction for securities sold during the period	0.0
Reduction for securities for which the Company made the decision to sell where an allowance was previously recorded	0.0
Write-offs	0.0
Recoveries of previously written-off amount	0.0
Allowance at the end of the period	$ 0.1