INVESTMENTS - SUMMARY OF CARRYING VALUE AND ESTIMATED FAIR VALUE OF OUTSTANDING COMMERCIAL MORTGAGE LOANS AND UNDERLYING COLLATERAL (Details) - Commercial Portfolio Segment $ in Millions	Jun. 30, 2020 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 42.1
2019	135.0
2018	156.5
2017	106.9
2016	113.4
Prior	819.9
Total amortized cost	1,373.8
Estimated fair value	1,381.5
Collateral	3,405.6
Less than 60%
Financing Receivable, Credit Quality Indicator [Line Items]
2020	23.0
2019	111.2
2018	132.5
2017	103.1
2016	56.9
Prior	636.2
Total amortized cost	1,062.9
Estimated fair value	1,079.2
Collateral	2,953.0
60% to less than 70%
Financing Receivable, Credit Quality Indicator [Line Items]
2020	19.1
2019	11.5
2018	24.0
2017	3.8
2016	46.5
Prior	84.6
Total amortized cost	189.5
Estimated fair value	187.6
Collateral	296.8
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	73.0
Total amortized cost	85.3
Estimated fair value	83.9
Collateral	114.1
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	26.1
Total amortized cost	36.1
Estimated fair value	30.8
Collateral	$ 41.7