FAIR VALUE OF FINANCIAL INSTRUMENTS - Schedule of Quantitative Information (Details) - Discounted Cash Flow - Level 3 $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 1,106,517	$ 2,100,947
CMBS
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 953,499	$ 1,701,395
CMBS | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	0.61%	1.35%
Duration (in years)	1 month 13 days	15 days
CMBS | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	3.00%	2.87%
Duration (in years)	3 years 2 months 8 days	3 years 6 months 18 days
CMBS | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	18.32%	9.18%
Duration (in years)	7 years 10 months 2 days	9 years 3 days
CMBS interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 113,072	$ 342,171
CMBS interest-only | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.70%	2.84%
Duration (in years)	4 months 20 days	0 years
Prepayment speed	100.00	100.00
CMBS interest-only | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	3.52%	4.04%
Duration (in years)	3 years 21 days	2 years 11 months 26 days
Prepayment speed	100.00	100.00
CMBS interest-only | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.31%	4.80%
Duration (in years)	4 years 5 months 15 days	4 years 4 months 13 days
Prepayment speed	100.00	100.00
GNMA interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 4,476	$ 16,821
GNMA interest-only | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.46%	0.87%
Duration (in years)	5 months 8 days	1 year 8 months 8 days
Prepayment speed	5.00	5.00
GNMA interest-only | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	11.85%	7.22%
Duration (in years)	2 years 5 months 4 days	4 years 5 months 8 days
Prepayment speed	12.19	13.80
GNMA interest-only | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	71.88%	48.64%
Duration (in years)	5 years 2 months 8 days	20 years 7 months 27 days
Prepayment speed	35.00	35.00
Agency securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 728	$ 780
Agency securities | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	1.40%	1.40%
Duration (in years)	0 years	2 years 7 months 9 days
Agency securities | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.16%	2.17%
Duration (in years)	3 years 2 months 19 days	3 years 3 months 7 days
Agency securities | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.52%	2.63%
Duration (in years)	4 years 8 months 19 days	4 years 4 months 20 days
GNMA permanent securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 34,742	$ 39,780
GNMA permanent securities | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.62%	2.63%
Duration (in years)	1 year 4 months 24 days	1 year 11 months 1 day
GNMA permanent securities | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	3.44%	3.65%
Duration (in years)	5 years 9 months	10 years 3 months 18 days
GNMA permanent securities | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.93%	6.92%
Duration (in years)	5 years 11 months 8 days	15 years 7 months 27 days