COMMERCIAL MORTGAGE LOANS - Schedule of Key Quality Indicators (Details) - Commercial Mortgage Loans $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized Cost	$ 10,227,726	$ 9,384,284
Financing Receivable, Before Allowance For Credit Loss | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Percentage of Total	100.00%	100.00%
Weighted Average
Financing Receivable, Credit Quality Indicator [Line Items]
Loan-to-value ratio with participating interest (as a percent)	54.00%	54.00%
Debt service coverage ratio	1.72	1.73
Greater than 75%
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized Cost	$ 399,384	$ 237,881
Greater than 75% | Financing Receivable, Before Allowance For Credit Loss | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Percentage of Total	3.90%	2.50%
Greater than 75% | Weighted Average
Financing Receivable, Credit Quality Indicator [Line Items]
Debt service coverage ratio	0.05	0.03
50% - 75%
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized Cost	$ 6,557,292	$ 6,074,051
50% - 75% | Financing Receivable, Before Allowance For Credit Loss | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Percentage of Total	64.10%	64.70%
50% - 75% | Weighted Average
Financing Receivable, Credit Quality Indicator [Line Items]
Debt service coverage ratio	1.04	1.07
Less than 50%
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized Cost	$ 3,271,050	$ 3,072,352
Less than 50% | Financing Receivable, Before Allowance For Credit Loss | Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Percentage of Total	32.00%	32.80%
Less than 50% | Weighted Average
Financing Receivable, Credit Quality Indicator [Line Items]
Debt service coverage ratio	0.63	0.63