REAL ESTATE SECURITIES - Summary of Securities (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) securities	Dec. 31, 2015 USD ($) securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 11,356,252	$ 10,084,386
Amortized Cost Basis	2,606,165	2,415,727
Gross Unrealized Gains	48,434	6,936
Gross Unrealized Losses	(3,640)	(15,446)
Carrying Value	$ 2,650,959	$ 2,407,217
Number of Securities | securities	230	200
Weighted Average Coupon %	1.30%	1.44%
Weighted Average Yield %	2.91%	3.60%
Remaining Duration (years)	3 years 2 months 12 days	3 years 3 months 14 days
CMBS
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 2,169,241	$ 1,972,492
Amortized Cost Basis	2,199,698	1,994,928
Gross Unrealized Gains	42,550	4,643
Gross Unrealized Losses	(855)	(8,065)
Carrying Value	$ 2,241,393	$ 1,991,506
Number of Securities | securities	142	119
Weighted Average Coupon %	3.24%	3.17%
Weighted Average Yield %	2.79%	2.59%
Remaining Duration (years)	3 years 21 days	3 years 1 month 24 days
CMBS interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 8,626,247	$ 7,436,379
Amortized Cost Basis	343,218	348,222
Gross Unrealized Gains	4,589	1,027
Gross Unrealized Losses	(440)	(4,826)
Carrying Value	$ 347,367	$ 344,423
Number of Securities | securities	57	48
Weighted Average Coupon %	0.83%	1.02%
Weighted Average Yield %	3.52%	3.81%
Remaining Duration (years)	3 years 1 month 20 days	3 years 4 months 2 days
GNMA interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 519,846	$ 632,175
Amortized Cost Basis	21,444	28,311
Gross Unrealized Gains	102	44
Gross Unrealized Losses	(2,190)	(2,161)
Carrying Value	$ 19,356	$ 26,194
Number of Securities | securities	18	20
Weighted Average Coupon %	0.74%	0.80%
Weighted Average Yield %	4.02%	4.26%
Remaining Duration (years)	4 years 7 months 27 days	5 years 2 months 19 days
GNMA construction securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount		$ 27,091
Amortized Cost Basis		27,581
Gross Unrealized Gains		1,058
Gross Unrealized Losses		0
Carrying Value		$ 28,639
Number of Securities | securities		1
Weighted Average Coupon %		4.10%
Weighted Average Yield %		3.86%
Remaining Duration (years)		9 years 3 months 29 days
Agency securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 798
Amortized Cost Basis	828
Gross Unrealized Gains	3
Gross Unrealized Losses	(1)
Carrying Value	$ 830
Number of Securities | securities	2
Weighted Average Coupon %	2.93%
Weighted Average Yield %	1.29%
Remaining Duration (years)	3 years 5 months 4 days
GNMA permanent securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 40,120	$ 16,249
Amortized Cost Basis	40,977	16,685
Gross Unrealized Gains	1,190	164
Gross Unrealized Losses	(154)	(394)
Carrying Value	$ 42,013	$ 16,455
Number of Securities | securities	11	12
Weighted Average Coupon %	4.16%	4.52%
Weighted Average Yield %	3.85%	3.94%
Remaining Duration (years)	10 years 1 month 9 days	5 years 5 months 4 days