INVESTMENTS - SUMMARY OF CARRYING VALUE AND ESTIMATED FAIR VALUE OF OUTSTANDING COMMERCIAL MORTGAGE LOANS AND UNDERLYING COLLATERAL (Details) - Commercial Portfolio Segment $ in Millions	Sep. 30, 2020 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 44.6
2019	134.5
2018	155.6
2017	106.2
2016	112.8
Prior	811.2
Total amortized cost	1,364.9
Estimated fair value	1,404.2
Collateral	3,422.5
Less than 60%
Financing Receivable, Credit Quality Indicator [Line Items]
2020	25.6
2019	114.9
2018	131.7
2017	102.4
2016	56.6
Prior	630.7
Total amortized cost	1,061.9
Estimated fair value	1,105.1
Collateral	2,971.2
60% to less than 70%
Financing Receivable, Credit Quality Indicator [Line Items]
2020	19.0
2019	7.3
2018	23.9
2017	3.8
2016	46.2
Prior	110.0
Total amortized cost	210.2
Estimated fair value	209.7
Collateral	331.5
70% to less than 80%
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0.0
2019	12.3
2018	0.0
2017	0.0
2016	0.0
Prior	44.5
Total amortized cost	56.8
Estimated fair value	56.7
Collateral	76.9
80% to less than 90%
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0.0
2019	0.0
2018	0.0
2017	0.0
2016	10.0
Prior	26.0
Total amortized cost	36.0
Estimated fair value	32.7
Collateral	$ 42.9