VARIABLE INTEREST ENTITIES - Geographic Concentrations of Credit Risk Exceeding 5% of Total Loan Unpaid Principal Balances (Details) - Consolidated VIEs $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Commercial Trusts
Concentration Risk [Line Items]
Principal balance	$ 3,287,085
Commercial Trusts | California
Concentration Risk [Line Items]
Principal balance	1,270,650
Commercial Trusts | Texas
Concentration Risk [Line Items]
Principal balance	478,048
Commercial Trusts | New York
Concentration Risk [Line Items]
Principal balance	353,800
Commercial Trusts | Other
Concentration Risk [Line Items]
Principal balance	$ 1,184,587
Commercial Trusts | Securitized Loans | Geographic Concentration Risk
Concentration Risk [Line Items]
% of balance	100.00%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | California
Concentration Risk [Line Items]
% of balance	38.70%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | Texas
Concentration Risk [Line Items]
% of balance	14.50%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | New York
Concentration Risk [Line Items]
% of balance	10.80%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | Other
Concentration Risk [Line Items]
% of balance	36.00%
Residential Trusts
Concentration Risk [Line Items]
Principal balance	$ 75,382
Residential Trusts | California
Concentration Risk [Line Items]
Principal balance	34,578
Residential Trusts | Texas
Concentration Risk [Line Items]
Principal balance	10,116
Residential Trusts | Illinois
Concentration Risk [Line Items]
Principal balance	7,055
Residential Trusts | Washington
Concentration Risk [Line Items]
Principal balance	3,880
Residential Trusts | Other
Concentration Risk [Line Items]
Principal balance	$ 19,753
Residential Trusts | Securitized Loans | Geographic Concentration Risk
Concentration Risk [Line Items]
% of balance	100.00%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | California
Concentration Risk [Line Items]
% of balance	45.90%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | Texas
Concentration Risk [Line Items]
% of balance	13.40%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | Illinois
Concentration Risk [Line Items]
% of balance	9.40%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | Washington
Concentration Risk [Line Items]
% of balance	5.10%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | Other
Concentration Risk [Line Items]
% of balance	26.20%