FAIR VALUE OF FINANCIAL INSTRUMENTS - Unobservable Inputs (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
SBA 7(a) loans receivable, subject to loan-backed notes | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Rate of prepayment	9.59%	9.59%
SBA 7(a) loans receivable, subject to loan-backed notes | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Rate of prepayment	17.50%	17.50%
SBA 7(a) loans receivable, subject to credit risk | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Rate of prepayment	4.91%	4.91%
SBA 7(a) loans receivable, subject to credit risk | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Rate of prepayment	17.50%	17.50%
SBA 7(a) loans receivable, subject to secured borrowings | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Rate of prepayment	10.29%	10.29%
SBA 7(a) loans receivable, subject to secured borrowings | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Rate of prepayment	17.50%	17.50%
Measurement Input, Discount Rate | Mortgages Payable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for debt	0.0427
Measurement Input, Discount Rate | Mortgages Payable | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for debt		0.0462
Measurement Input, Discount Rate | Mortgages Payable | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for debt		0.0464
Measurement Input, Discount Rate | Junior Subordinated Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for debt	0.0685	0.0705
Measurement Input, Discount Rate | SBA 7(a) loans receivable, subject to loan-backed notes | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for loans receivable	0.0775	0.0675
Measurement Input, Discount Rate | SBA 7(a) loans receivable, subject to loan-backed notes | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for loans receivable	0.0950	0.0925
Measurement Input, Discount Rate | SBA 7(a) loans receivable, subject to credit risk | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for loans receivable	0.0725	0.0675
Measurement Input, Discount Rate | SBA 7(a) loans receivable, subject to credit risk | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for loans receivable	0.1000	0.0975
Measurement Input, Discount Rate | SBA 7(a) loans receivable, subject to secured borrowings | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for loans receivable	0.0900	0.0875
Measurement Input, Discount Rate | SBA 7(a) loans receivable, subject to secured borrowings | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for loans receivable	0.0975	0.0950