REAL ESTATE SECURITIES - Summary of Securities (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) securities	Dec. 31, 2016 USD ($) securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 4,556,934	$ 10,354,816
Amortized Cost Basis	1,093,784	2,100,994
Gross Unrealized Gains	8,715	13,194
Gross Unrealized Losses	(4,028)	(13,241)
Carrying Value	$ 1,098,471	$ 2,100,947
Number of Securities | securities	140	219
Weighted Average Coupon %	1.28%	1.27%
Weighted Average Yield %	2.82%	2.94%
Remaining Duration (years)	2 years 11 months 15 days	3 years 7 months 6 days
CMBS
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 934,559	$ 1,676,680
Amortized Cost Basis	946,231	1,698,616
Gross Unrealized Gains	6,195	10,880
Gross Unrealized Losses	(2,397)	(8,101)
Carrying Value	$ 950,029	$ 1,701,395
Number of Securities | securities	89	131
Weighted Average Coupon %	3.24%	3.26%
Weighted Average Yield %	2.73%	2.81%
Remaining Duration (years)	2 years 10 months 6 days	3 years 6 months 18 days
CMBS interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 3,308,063	$ 8,160,458
Amortized Cost Basis	102,986	343,438
Gross Unrealized Gains	1,542	1,273
Gross Unrealized Losses	(45)	(2,540)
Carrying Value	$ 104,483	$ 342,171
Number of Securities | securities	27	60
Weighted Average Coupon %	0.75%	0.87%
Weighted Average Yield %	3.14%	3.45%
Remaining Duration (years)	2 years 10 months 17 days	2 years 11 months 26 days
GNMA interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 279,567	$ 478,577
Amortized Cost Basis	9,138	18,994
Gross Unrealized Gains	188	159
Gross Unrealized Losses	(1,327)	(2,332)
Carrying Value	$ 7,999	$ 16,821
Number of Securities | securities	16	17
Weighted Average Coupon %	0.60%	0.73%
Weighted Average Yield %	5.99%	4.19%
Remaining Duration (years)	4 years 4 months 9 days	4 years 5 months 8 days
Agency securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 731	$ 774
Amortized Cost Basis	754	802
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(12)	(22)
Carrying Value	$ 742	$ 780
Number of Securities | securities	2	2
Weighted Average Coupon %	2.84%	2.90%
Weighted Average Yield %	1.82%	1.29%
Remaining Duration (years)	3 years 29 days	3 years 3 months 7 days
GNMA permanent securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 34,014	$ 38,327
Amortized Cost Basis	34,675	39,144
Gross Unrealized Gains	790	882
Gross Unrealized Losses	(247)	(246)
Carrying Value	$ 35,218	$ 39,780
Number of Securities | securities	6	9
Weighted Average Coupon %	3.98%	4.09%
Weighted Average Yield %	3.63%	3.80%
Remaining Duration (years)	5 years 9 months 25 days	10 years 3 months 18 days