REAL ESTATE SECURITIES - Summary of Securities (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) securities	Dec. 31, 2016 USD ($) securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 9,604,409	$ 10,354,816
Amortized Cost Basis	1,697,117	2,100,994
Gross Unrealized Gains	13,733	13,194
Gross Unrealized Losses	(8,870)	(13,241)
Carrying Value	$ 1,701,980	$ 2,100,947
Number of Securities | securities	215	219
Weighted Average Coupon %	1.31%	1.27%
Weighted Average Yield %	2.92%	2.94%
Remaining Duration (years)	3 years 4 months 17 days	3 years 7 months 6 days
CMBS
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 1,306,653	$ 1,676,680
Amortized Cost Basis	1,323,369	1,698,616
Gross Unrealized Gains	7,909	10,880
Gross Unrealized Losses	(6,028)	(8,101)
Carrying Value	$ 1,325,250	$ 1,701,395
Number of Securities | securities	128	131
Weighted Average Coupon %	3.18%	3.26%
Weighted Average Yield %	2.72%	2.81%
Remaining Duration (years)	3 years 4 months 17 days	3 years 6 months 18 days
CMBS interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 7,801,370	$ 8,160,458
Amortized Cost Basis	316,486	343,438
Gross Unrealized Gains	4,749	1,273
Gross Unrealized Losses	(370)	(2,540)
Carrying Value	$ 320,865	$ 342,171
Number of Securities | securities	59	60
Weighted Average Coupon %	1.01%	0.87%
Weighted Average Yield %	3.58%	3.45%
Remaining Duration (years)	3 years 14 days	2 years 11 months 26 days
GNMA interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 457,597	$ 478,577
Amortized Cost Basis	17,640	18,994
Gross Unrealized Gains	196	159
Gross Unrealized Losses	(2,211)	(2,332)
Carrying Value	$ 15,625	$ 16,821
Number of Securities | securities	17	17
Weighted Average Coupon %	0.72%	0.73%
Weighted Average Yield %	4.47%	4.19%
Remaining Duration (years)	4 years 4 months 13 days	4 years 5 months 8 days
Agency securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 760	$ 774
Amortized Cost Basis	786	802
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(15)	(22)
Carrying Value	$ 771	$ 780
Number of Securities | securities	2	2
Weighted Average Coupon %	2.88%	2.90%
Weighted Average Yield %	1.90%	1.29%
Remaining Duration (years)	3 years 4 months 6 days	3 years 3 months 7 days
GNMA permanent securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 38,029	$ 38,327
Amortized Cost Basis	38,836	39,144
Gross Unrealized Gains	879	882
Gross Unrealized Losses	(246)	(246)
Carrying Value	$ 39,469	$ 39,780
Number of Securities | securities	9	9
Weighted Average Coupon %	4.08%	4.09%
Weighted Average Yield %	3.69%	3.80%
Remaining Duration (years)	5 years 9 months 21 days	10 years 3 months 18 days