INVESTMENTS - SUMMARY OF CHANGES IN THE ALLOWANCE FOR CURRENT EXPECTED CREDIT LOSSES (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning allowance	$ 19.9	$ 2.1
Additions for securities for which credit losses were not previously recorded	4.5	23.7
Additions for purchased securities with deteriorated credit	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	(13.1)	(14.2)
Reduction for securities sold during the period	(0.5)	(0.8)
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
Ending allowance	10.8	10.8
Corporate securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning allowance	18.2	2.1
Additions for securities for which credit losses were not previously recorded	4.2	21.7
Additions for purchased securities with deteriorated credit	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	(11.9)	(13.0)
Reduction for securities sold during the period	(0.5)	(0.8)
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
Ending allowance	10.0	10.0
States and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning allowance	0.6	0.0
Additions for securities for which credit losses were not previously recorded	0.0	0.6
Additions for purchased securities with deteriorated credit	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	(0.1)	(0.1)
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
Ending allowance	0.5	0.5
Foreign governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning allowance	0.1	0.0
Additions for securities for which credit losses were not previously recorded	0.0	0.1
Additions for purchased securities with deteriorated credit	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	(0.1)	(0.1)
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
Ending allowance	0.0	0.0
Non-agency residential mortgage-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning allowance	1.0	0.0
Additions for securities for which credit losses were not previously recorded	0.0	1.0
Additions for purchased securities with deteriorated credit	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	(1.0)	(1.0)
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
Ending allowance	0.0	0.0
Asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning allowance	0.0	0.0
Additions for securities for which credit losses were not previously recorded	0.3	0.3
Additions for purchased securities with deteriorated credit	0.0	0.0
Additions (reductions) for securities where an allowance was previously recorded	0.0	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
Ending allowance	$ 0.3	$ 0.3

Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

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