INVESTMENTS - SUMMARY OF CHANGES IN THE ALLOWANCE FOR CURRENT EXPECTED CREDIT LOSSES (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance at the beginning of the period	$ 7.6	$ 2.2	$ 2.1
Additions for securities for which credit losses were not previously recorded	50.4	6.3	25.7
Additions for purchased securities with deteriorated credit	0.0	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	10.3	0.0	(24.1)
Reduction for securities sold during the period	(12.3)	(0.9)	(1.5)
Reduction for securities for which the Company made the decision to sell where an allowance was previously recorded	0.0	0.0	0.0
Write-offs	0.0	0.0	0.0
Recoveries of previously written-off amount	0.0	0.0	0.0
Allowance at the end of the period	56.0	7.6	2.2
Corporate securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance at the beginning of the period	7.4	1.9	2.1
Additions for securities for which credit losses were not previously recorded	48.9	6.1	23.6
Additions for purchased securities with deteriorated credit	0.0	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	10.3	0.2	(22.3)
Reduction for securities sold during the period	(12.2)	(0.8)	(1.5)
Reduction for securities for which the Company made the decision to sell where an allowance was previously recorded	0.0	0.0	0.0
Write-offs	0.0	0.0	0.0
Recoveries of previously written-off amount	0.0	0.0	0.0
Allowance at the end of the period	54.4	7.4	1.9
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	0.0
Additions for securities for which credit losses were not previously recorded	0.7	0.1	0.7
Additions for purchased securities with deteriorated credit	0.0	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	0.3	(0.4)	(0.4)
Reduction for securities sold during the period	(0.1)	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	0.0
Write-offs	0.0	0.0	0.0
Recoveries of previously written-off amount	0.0	0.0	0.0
Allowance at the end of the period	0.9	0.0	0.3
Foreign governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance at the beginning of the period	0.2	0.0	0.0
Additions for securities for which credit losses were not previously recorded	0.5	0.1	0.1
Additions for purchased securities with deteriorated credit	0.0	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	(0.3)	0.2	(0.1)
Reduction for securities sold during the period	0.0	(0.1)	0.0
Reduction for securities for which the Company made the decision to sell where an allowance was previously recorded	0.0	0.0	0.0
Write-offs	0.0	0.0	0.0
Recoveries of previously written-off amount	0.0	0.0	0.0
Allowance at the end of the period	0.4	0.2	0.0
Asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance at the beginning of the period	0.0	0.0	0.0
Additions for securities for which credit losses were not previously recorded	0.3	0.0	0.3
Additions for purchased securities with deteriorated credit	0.0	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	0.0	0.0	(0.3)
Reduction for securities sold during the period	0.0	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	0.0
Write-offs	0.0	0.0	0.0
Recoveries of previously written-off amount	0.0	0.0	0.0
Allowance at the end of the period	0.3	0.0	0.0
Non-agency residential mortgage-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance at the beginning of the period	0.0	0.0	0.0
Additions for securities for which credit losses were not previously recorded	0.0	0.0	1.0
Additions for purchased securities with deteriorated credit	0.0	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	0.0	0.0	(1.0)
Reduction for securities sold during the period	0.0	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	0.0
Write-offs	0.0	0.0	0.0
Recoveries of previously written-off amount	0.0	0.0	0.0
Allowance at the end of the period	$ 0.0	$ 0.0	$ 0.0