VARIABLE INTEREST ENTITIES - Geographic Concentrations of Credit Risk Exceeding 5% of Total Loan Unpaid Principal Balances (Details) - Consolidated VIEs $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)
Commercial Trusts
Concentration Risk [Line Items]
Principal Balance
Commercial Trusts | California
Concentration Risk [Line Items]
Principal Balance	492,586
Commercial Trusts | California
Concentration Risk [Line Items]
Principal Balance	473,786
Commercial Trusts | Florida
Concentration Risk [Line Items]
Principal Balance	143,025
Commercial Trusts | Florida
Concentration Risk [Line Items]
Principal Balance	1,058,775
Commercial Trusts | Total
Concentration Risk [Line Items]
Principal Balance	$ 2,168,172
Commercial Trusts | Securitized Loans | Geographic Concentration Risk
Concentration Risk [Line Items]
% of Balance
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | California
Concentration Risk [Line Items]
% of Balance	22.70%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | California
Concentration Risk [Line Items]
% of Balance	21.90%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | Florida
Concentration Risk [Line Items]
% of Balance	6.60%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | Florida
Concentration Risk [Line Items]
% of Balance	48.80%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | Total
Concentration Risk [Line Items]
% of Balance	100.00%
Residential Trusts
Concentration Risk [Line Items]
Principal Balance	$ 93,921
Residential Trusts | California
Concentration Risk [Line Items]
Principal Balance	13,168
Residential Trusts | California
Concentration Risk [Line Items]
Principal Balance	39,860
Residential Trusts | Illinois
Concentration Risk [Line Items]
Principal Balance	7,148
Residential Trusts | Washington
Concentration Risk [Line Items]
Principal Balance	6,452
Residential Trusts | Other (1)
Concentration Risk [Line Items]
Principal Balance	$ 27,293
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	14.00%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | California
Concentration Risk [Line Items]
% of Balance	42.40%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | Illinois
Concentration Risk [Line Items]
% of Balance	7.60%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | Washington
Concentration Risk [Line Items]
% of Balance	6.90%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | Other (1)
Concentration Risk [Line Items]
% of Balance	29.10%