COMMERCIAL MORTGAGE LOANS HELD-FOR-INVESTMENT - Summary of Concentration of Credit Risk (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	100.00%	100.00%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	100.00%	100.00%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Multi-Family Property [Member] | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	83.80%	87.20%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Office Property [Member] | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	7.30%	7.60%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Retail Property [Member] | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	7.20%	1.20%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Self-Storage [Member] | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	0.90%	1.00%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Mixed Use Property [Member] | Collateral Property Type
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	0.80%	3.00%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Southwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	36.30%	30.20%
Commercial loans held-for-investment | Commercial real estate portfolio segment | South | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	21.60%	22.60%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Midwest | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	17.50%	20.20%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Mid-Atlantic | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	11.30%	10.30%
Commercial loans held-for-investment | Commercial real estate portfolio segment | West | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	7.70%	10.80%
Commercial loans held-for-investment | Commercial real estate portfolio segment | Various | Geography
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	5.60%	5.90%