LOANS - Additional Information about Residential Mortgage Loans (Details) - Residential Mortgage Loans $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($) point	Dec. 31, 2019 USD ($) point
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 4,372,834	$ 4,133,149
Minimum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 0	$ 1
Interest rate	2.00%	2.00%
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	8.75%	8.38%
FICO score at loan origination | point	829	829
Loan-to-value ratio at loan origination	105.00%	105.00%
Weighted Average
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 451	$ 459
Interest rate	4.89%	4.94%
FICO score at loan origination | point	759	758
Loan-to-value ratio at loan origination	67.00%	67.00%