FAIR VALUE OF FINANCIAL INSTRUMENTS - Unobservable Inputs (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
SBA 7(a) loans receivable, subject to loan-backed notes | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value inputs, prepayment rates	7.30%
SBA 7(a) loans receivable, subject to loan-backed notes | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value inputs, prepayment rates	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]
Fair value inputs, prepayment rates	15.50%	15.50%
SBA 7(a) loans receivable, subject to secured borrowings | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value inputs, prepayment rates	17.50%	17.50%
Loans Receivable Subject to Credit Risk | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value inputs, prepayment rates	7.30%	7.30%
Loans Receivable Subject to Credit Risk | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value inputs, prepayment rates	17.50%	17.50%
Discount rate | Mortgages Payable | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for debt	0.0475	0.0415
Discount rate | Mortgages Payable | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for debt	0.0476	0.0428
Discount rate | Junior Subordinated Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input for debt	0.0665	0.0594
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	0.0825
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	0.0975
Discount rate | 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	0.0750	0.0625
Discount rate | 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	0.0975	0.0900