LOANS - Summary of Sector Attributes of Commercial Real Estate Investments (Details) - Commercial Mortgage - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	$ 1,384,122	$ 1,372,430	$ 1,618,794
Percentage of Loan Portfolio	100.00%	100.00%
Office
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	$ 577,893	$ 650,034
Percentage of Loan Portfolio	41.80%	47.40%
Retail
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	$ 249,917	$ 256,493
Percentage of Loan Portfolio	18.10%	18.70%
Multifamily
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	$ 217,131	$ 250,095
Percentage of Loan Portfolio	15.70%	18.20%
Hotel
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	$ 130,578	$ 115,536
Percentage of Loan Portfolio	9.40%	8.40%
Industrial
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	$ 114,152	$ 60,097
Percentage of Loan Portfolio	8.20%	4.40%
Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	$ 75,306	$ 20,302
Percentage of Loan Portfolio	5.40%	1.50%
Healthcare
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans	$ 19,145	$ 19,873
Percentage of Loan Portfolio	1.40%	1.40%