COMMERCIAL MORTGAGE LOANS - Additional Information (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) loan	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Commercial mortgage loans on real estate, net	$ 10,137,000		$ 10,006,000
Amount that would become due in remainder of current fiscal year, if loans are called at their next call dates	168,000
Amount that would become due in 2022 through 2026, if loans are called at their next call dates	538,000
Amount that would become due in 2027 through 2029, if loans are called at their next call dates	10,000
Increase in allowance for credit loss on mortgage loans due to deterioration in macroeconomic forecasts as a result of COVID-19	58,000
Commercial Mortgage Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Commercial mortgage loans, before allowance for credit losses	10,308,000		10,228,000
Commercial mortgage loans on real estate, net	10,137,000		10,006,000
Mortgage loans having participation feature	774,000		806,000
Income recognized on participating mortgage loans	7,000	$ 16,000
Loans not subject to pooling and servicing agreement, non performing or restructured amount	$ 1,000		$ 3,000
Non-accrual status, no related allowance, number of loans | loan	1		1
Non-accrual status, no related allowance, average recorded investment	$ 1,000		$ 1,000
Non-accrual status, no related allowance, recorded investment	1,000		1,000
Non-accrual status, no related allowance, unpaid principal	$ 1,000		$ 1,000
Commercial Mortgage Loans | 90 Days and Greater Delinquent
Financing Receivable, Allowance for Credit Loss [Line Items]
Number of delinquent loans | loan	1
Value of loans in delinquency	$ 1,000
Commercial Mortgage Loans | 60-89 Days Delinquent
Financing Receivable, Allowance for Credit Loss [Line Items]
Number of delinquent loans | loan			1
Value of loans in delinquency			$ 1,000
Commercial Mortgage Loans | CARES Act, Loan Modification Program
Financing Receivable, Allowance for Credit Loss [Line Items]
Commercial mortgage loans, before allowance for credit losses	$ 2,200,000
Number of loans modified and not considered troubled debt restructuring | loan	7
Loans modified during period and not considered troubled debt restructuring	$ 143,000
Number of loans in modification and not considered troubled debt restructuring | loan	295
Commercial Mortgage Loans | CARES Act, Not In Payment Deferral, Loan Modification Program
Financing Receivable, Allowance for Credit Loss [Line Items]
Commercial mortgage loans, before allowance for credit losses	$ 1,700,000
Maximum | Commercial Mortgage Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Mortgage call option period	9 years
Loan-to-value ratio with participating interest (as a percent)	85.00%