LOANS - Additional Data On Residential Mortgage Loans (Details) - Residential Mortgage Loans $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021 USD ($) point	Dec. 31, 2020 USD ($) point
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 3,557,855	$ 3,482,865
Minimum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 1	$ 1
Interest rate	0.50%	0.50%
FICO score at loan origination | point	505	505
Loan-to-value ratio at loan origination	8.00%	8.00%
Maximum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 3,448	$ 3,448
Interest rate	9.24%	9.24%
FICO score at loan origination | point	829	829
Loan-to-value ratio at loan origination	104.00%	104.00%
Weighted Average
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 485	$ 473
Interest rate	4.85%	4.89%
FICO score at loan origination | point	754	755
Loan-to-value ratio at loan origination	67.00%	67.00%