FAIR VALUE OF FINANCIAL INSTRUMENTS - Schedule of Quantitative Information (Details) - Discounted Cash Flow - Level 3 - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 1,098,471	$ 2,100,947
CMBS
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 950,029	$ 1,701,395
CMBS | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	0.00%	1.35%
Duration (in years)	0 days	15 days
CMBS | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.49%	2.87%
Duration (in years)	3 years 5 months 4 days	3 years 6 months 18 days
CMBS | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.31%	9.18%
Duration (in years)	9 years 6 months 7 days	9 years 3 days
CMBS interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 104,483	$ 342,171
CMBS interest-only | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	1.35%	2.84%
Duration (in years)	11 months 9 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)	2.56%	4.04%
Duration (in years)	5 years 2 months 1 day	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)	4.59%	4.80%
Duration (in years)	9 years 4 months 9 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	$ 7,999	$ 16,821
GNMA interest-only | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	0.00%	0.87%
Duration (in years)	0 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)	9.38%	7.22%
Duration (in years)	5 years 7 months 17 days	4 years 5 months 8 days
Prepayment speed	16.41	13.80
GNMA interest-only | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	10.00%	48.64%
Duration (in years)	9 years 9 months 10 days	20 years 7 months 28 days
Prepayment speed	35.00	35.00
Agency securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 742	$ 780
Agency securities | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.26%	1.40%
Duration (in years)	2 years 8 months 12 days	2 years 7 months 9 days
Agency securities | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.39%	2.17%
Duration (in years)	4 years 5 months 15 days	3 years 3 months 7 days
Agency securities | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.66%	2.63%
Duration (in years)	5 years 3 months 21 days	4 years 4 months 20 days
GNMA permanent securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 35,218	$ 39,780
GNMA permanent securities | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.64%	2.63%
Duration (in years)	1 year 10 months 2 days	1 year 11 months 1 day
GNMA permanent securities | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	3.34%	3.65%
Duration (in years)	7 years 7 days	10 years 3 months 18 days
GNMA permanent securities | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.59%	6.92%
Duration (in years)	7 years 3 months 14 days	15 years 7 months 28 days