VARIABLE INTEREST ENTITIES - Geographic Concentrations of Credit Risk Exceeding 5% of Total Loan Unpaid Principal Balances (Details) - Consolidated VIEs $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Commercial Trusts
Concentration Risk [Line Items]
Principal Balance	$ 3,050,513
Commercial Trusts | Texas
Concentration Risk [Line Items]
Principal Balance	546,190
Commercial Trusts | California
Concentration Risk [Line Items]
Principal Balance	461,478
Commercial Trusts | Maryland
Concentration Risk [Line Items]
Principal Balance	407,266
Commercial Trusts | Maryland
Concentration Risk [Line Items]
Principal Balance	349,921
Commercial Trusts | Pennsylvania
Concentration Risk [Line Items]
Principal Balance	280,201
Commercial Trusts | Other
Concentration Risk [Line Items]
Principal Balance	$ 1,005,457
Commercial Trusts | Securitized Loans | Geographic Concentration Risk
Concentration Risk [Line Items]
% of Balance	100.00%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | Texas
Concentration Risk [Line Items]
% of Balance	17.90%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | California
Concentration Risk [Line Items]
% of Balance	15.10%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | Maryland
Concentration Risk [Line Items]
% of Balance	13.40%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | Maryland
Concentration Risk [Line Items]
% of Balance	11.50%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | Pennsylvania
Concentration Risk [Line Items]
% of Balance	9.20%
Commercial Trusts | Securitized Loans | Geographic Concentration Risk | Other
Concentration Risk [Line Items]
% of Balance	32.90%
Residential Trusts
Concentration Risk [Line Items]
Principal Balance	$ 101,267
Residential Trusts | Texas
Concentration Risk [Line Items]
Principal Balance	13,261
Residential Trusts | California
Concentration Risk [Line Items]
Principal Balance	45,023
Residential Trusts | Other
Concentration Risk [Line Items]
Principal Balance	23,155
Residential Trusts | Washington
Concentration Risk [Line Items]
Principal Balance	7,466
Residential Trusts | Illinois
Concentration Risk [Line Items]
Principal Balance	7,197
Residential Trusts | Florida
Concentration Risk [Line Items]
Principal Balance	$ 5,165
Residential Trusts | Securitized Loans | Geographic Concentration Risk
Concentration Risk [Line Items]
% of Balance	100.00%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | Texas
Concentration Risk [Line Items]
% of Balance	13.10%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | California
Concentration Risk [Line Items]
% of Balance	44.50%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | Other
Concentration Risk [Line Items]
% of Balance	22.80%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | Washington
Concentration Risk [Line Items]
% of Balance	7.40%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | Illinois
Concentration Risk [Line Items]
% of Balance	7.10%
Residential Trusts | Securitized Loans | Geographic Concentration Risk | Florida
Concentration Risk [Line Items]
% of Balance	5.10%