REAL ESTATE SECURITIES - Summary of Securities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) securities	Dec. 31, 2015 USD ($) securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 10,354,816	$ 10,084,386
Amortized Cost Basis	2,100,994	2,415,727
Gross Unrealized Gains	13,194	6,936
Gross Unrealized Losses	(13,241)	(15,446)
Carrying Value	$ 2,100,947	$ 2,407,217
Number of Securities | securities	219	200
Weighted Average Coupon %	1.27%	1.44%
Weighted Average Yield %	2.94%	3.60%
Remaining Duration (years)	3 years 7 months 6 days	3 years 3 months 14 days
CMBS
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 1,676,680	$ 1,972,492
Amortized Cost Basis	1,698,616	1,994,928
Gross Unrealized Gains	10,880	4,643
Gross Unrealized Losses	(8,101)	(8,065)
Carrying Value	$ 1,701,395	$ 1,991,506
Number of Securities | securities	131	119
Weighted Average Coupon %	3.26%	3.17%
Weighted Average Yield %	2.81%	2.59%
Remaining Duration (years)	3 years 6 months 18 days	3 years 1 month 24 days
CMBS interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 8,160,458	$ 7,436,379
Amortized Cost Basis	343,438	348,222
Gross Unrealized Gains	1,273	1,027
Gross Unrealized Losses	(2,540)	(4,826)
Carrying Value	$ 342,171	$ 344,423
Number of Securities | securities	60	48
Weighted Average Coupon %	0.87%	1.02%
Weighted Average Yield %	3.45%	3.81%
Remaining Duration (years)	2 years 11 months 26 days	3 years 4 months 2 days
GNMA interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 478,577	$ 632,175
Amortized Cost Basis	18,994	28,311
Gross Unrealized Gains	159	44
Gross Unrealized Losses	(2,332)	(2,161)
Carrying Value	$ 16,821	$ 26,194
Number of Securities | securities	17	20
Weighted Average Coupon %	0.73%	0.80%
Weighted Average Yield %	4.19%	4.26%
Remaining Duration (years)	4 years 5 months 8 days	5 years 2 months 19 days
Agency securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 774
Amortized Cost Basis	802
Gross Unrealized Gains	0
Gross Unrealized Losses	(22)
Carrying Value	$ 780
Number of Securities | securities	2
Weighted Average Coupon %	2.90%
Weighted Average Yield %	1.29%
Remaining Duration (years)	3 years 3 months 7 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
GNMA permanent securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 38,327	$ 16,249
Amortized Cost Basis	39,144	16,685
Gross Unrealized Gains	882	164
Gross Unrealized Losses	(246)	(394)
Carrying Value	$ 39,780	$ 16,455
Number of Securities | securities	9	12
Weighted Average Coupon %	4.09%	4.52%
Weighted Average Yield %	3.80%	3.94%
Remaining Duration (years)	10 years 3 months 18 days	5 years 5 months 4 days