FAIR VALUE OF FINANCIAL INSTRUMENTS - Schedule of Quantitative Information (Details) - Discounted Cash Flow - Level 3 $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 2,650,959	$ 2,407,217
CMBS
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 2,241,393	$ 1,991,506
CMBS | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	(0.11%)	0.00%
Duration (in years)	0 years	0 years
CMBS | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.59%	2.19%
Duration (in years)	3 years 9 months 3 days	4 years 21 days
CMBS | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	19.14%	9.21%
Duration (in years)	9 years 1 month 24 days	7 years 10 months 27 days
CMBS interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 347,367	$ 344,423
CMBS interest-only | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.35%	0.09%
Duration (in years)	11 months 19 days	1 year 10 months 24 days
Prepayment speed	100.00	100.00
CMBS interest-only | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	3.40%	4.13%
Duration (in years)	3 years 1 month 13 days	3 years 3 months 18 days
Prepayment speed	100.00	100.00
CMBS interest-only | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	4.11%	4.51%
Duration (in years)	4 years 6 months 3 days	4 years 2 months 26 days
Prepayment speed	100.00	100.00
GNMA interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 19,356	$ 26,194
GNMA interest-only | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	(8.39%)	0.00%
Duration (in years)	1 month 17 days	3 months 25 days
Prepayment speed	5.00	5.00
GNMA interest-only | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.28%	9.21%
Duration (in years)	2 years 4 months 13 days	2 years 4 months 27 days
Prepayment speed	13.88	14.57
GNMA interest-only | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	45.01%	10.00%
Duration (in years)	6 years 8 months 1 day	5 years 2 months 4 days
Prepayment speed	35.00	35.00
Agency securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 830	$ 28,639
Agency securities | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	1.40%	0.58%
Duration (in years)	0 years	0 years
Agency securities | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	1.72%	3.47%
Duration (in years)	3 years 7 months 2 days	10 years 4 months 2 days
Agency securities | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	1.92%	3.51%
Duration (in years)	5 years 9 months 29 days	10 years 5 months 23 days
GNMA permanent securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Carrying Value	$ 42,013	$ 16,455
GNMA permanent securities | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	2.68%	0.00%
Duration (in years)	1 year 4 months 6 days	1 year 7 months 27 days
GNMA permanent securities | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	3.63%	3.25%
Duration (in years)	8 years 7 months 9 days	5 years 8 months 19 days
GNMA permanent securities | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Yield (as a percent)	6.76%	6.62%
Duration (in years)	10 years 1 month 27 days	7 years 2 months 15 days