COMMERCIAL MORTGAGE LOANS - Schedule of Key Quality Indicators (Details) - Commercial Mortgage Loans $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2021
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized Cost	$ 10,609	$ 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		1.72	1.73
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	$ 308	$ 399
Greater than 75% | Financing Receivable, Before Allowance For Credit Loss | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
% of Total	3.00%	4.00%
Greater than 75% | Weighted Average
Financing Receivable, Credit Quality Indicator [Line Items]
Debt service coverage ratio	1.32	1.29
50% - 75%
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized Cost	$ 6,946	$ 6,557
50% - 75% | Financing Receivable, Before Allowance For Credit Loss | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
% of Total	65.00%	64.00%
50% - 75% | Weighted Average
Financing Receivable, Credit Quality Indicator [Line Items]
Debt service coverage ratio	1.60	1.61
Less than 50%
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized Cost	$ 3,355	$ 3,272
Less than 50% | Financing Receivable, Before Allowance For Credit Loss | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
% of Total	32.00%	32.00%
Less than 50% | Weighted Average
Financing Receivable, Credit Quality Indicator [Line Items]
Debt service coverage ratio	2.03	2.01