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, 2020 USD ($)
Commercial Trusts
Concentration Risk [Line Items]
Principal balance	$ 3,241,472
Residential Trusts
Concentration Risk [Line Items]
Principal balance	39,459
California | Commercial Trusts
Concentration Risk [Line Items]
Principal balance	1,051,276
California | Residential Trusts
Concentration Risk [Line Items]
Principal balance	18,692
Illinois | Residential Trusts
Concentration Risk [Line Items]
Principal balance	5,356
Texas | Commercial Trusts
Concentration Risk [Line Items]
Principal balance	459,256
Texas | Residential Trusts
Concentration Risk [Line Items]
Principal balance	4,972
New York | Commercial Trusts
Concentration Risk [Line Items]
Principal balance	369,691
Florida | Commercial Trusts
Concentration Risk [Line Items]
Principal balance	196,865
Washington | Commercial Trusts
Concentration Risk [Line Items]
Principal balance	182,000
Arizona | Commercial Trusts
Concentration Risk [Line Items]
Principal balance	171,102
Massachusetts | Residential Trusts
Concentration Risk [Line Items]
Principal balance	2,265
Other | Commercial Trusts
Concentration Risk [Line Items]
Principal balance	811,282
Other | Residential Trusts
Concentration Risk [Line Items]
Principal balance	$ 8,174
Securitized Loans | Geographic Concentration Risk | Commercial Trusts
Concentration Risk [Line Items]
% of balance	100.00%
Securitized Loans | Geographic Concentration Risk | Residential Trusts
Concentration Risk [Line Items]
% of balance	100.00%
Securitized Loans | Geographic Concentration Risk | California | Commercial Trusts
Concentration Risk [Line Items]
% of balance	32.40%
Securitized Loans | Geographic Concentration Risk | California | Residential Trusts
Concentration Risk [Line Items]
% of balance	47.40%
Securitized Loans | Geographic Concentration Risk | Illinois | Residential Trusts
Concentration Risk [Line Items]
% of balance	13.60%
Securitized Loans | Geographic Concentration Risk | Texas | Commercial Trusts
Concentration Risk [Line Items]
% of balance	14.20%
Securitized Loans | Geographic Concentration Risk | Texas | Residential Trusts
Concentration Risk [Line Items]
% of balance	12.60%
Securitized Loans | Geographic Concentration Risk | New York | Commercial Trusts
Concentration Risk [Line Items]
% of balance	11.40%
Securitized Loans | Geographic Concentration Risk | Florida | Commercial Trusts
Concentration Risk [Line Items]
% of balance	6.10%
Securitized Loans | Geographic Concentration Risk | Washington | Commercial Trusts
Concentration Risk [Line Items]
% of balance	5.60%
Securitized Loans | Geographic Concentration Risk | Arizona | Commercial Trusts
Concentration Risk [Line Items]
% of balance	5.30%
Securitized Loans | Geographic Concentration Risk | Massachusetts | Residential Trusts
Concentration Risk [Line Items]
% of balance	5.70%
Securitized Loans | Geographic Concentration Risk | Other | Commercial Trusts
Concentration Risk [Line Items]
% of balance	25.00%
Securitized Loans | Geographic Concentration Risk | Other | Residential Trusts
Concentration Risk [Line Items]
% of balance	20.70%