COMMERCIAL MORTGAGE LOANS - Schedule of Key Quality Indicators (Details) - Commercial Mortgage Loans $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized Cost	$ 10,308	$ 10,228
Financing Receivable, Before Allowance For Credit Loss | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
% of Total	100.00%	100.00%
Weighted Average
Financing Receivable, Credit Quality Indicator [Line Items]
Debt service coverage ratio	1.74	1.72
Loan-to-value ratio with participating interest (as a percent)	54.00%	54.00%
Greater than 75%
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized Cost	$ 393	$ 399
Greater than 75% | Financing Receivable, Before Allowance For Credit Loss | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
% of Total	4.00%	4.00%
Greater than 75% | Weighted Average
Financing Receivable, Credit Quality Indicator [Line Items]
Debt service coverage ratio	0.05	0.05
50% - 75%
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized Cost	$ 6,504	$ 6,557
50% - 75% | Financing Receivable, Before Allowance For Credit Loss | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
% of Total	63.00%	64.00%
50% - 75% | Weighted Average
Financing Receivable, Credit Quality Indicator [Line Items]
Debt service coverage ratio	1.02	1.04
Less than 50%
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized Cost	$ 3,411	$ 3,272
Less than 50% | Financing Receivable, Before Allowance For Credit Loss | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
% of Total	33.00%	32.00%
Less than 50% | Weighted Average
Financing Receivable, Credit Quality Indicator [Line Items]
Debt service coverage ratio	0.67	0.63