REAL ESTATE SECURITIES - Summary of Securities (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($) securities	Dec. 31, 2015 USD ($) securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 10,477,607	$ 10,084,386
Amortized Cost Basis	2,646,793	2,415,727
Gross Unrealized Gains	57,740	6,936
Gross Unrealized Losses	(4,323)	(15,446)
Carrying Value	$ 2,700,210	$ 2,407,217
Number of Securities | securities	226	200
Weighted Average Coupon %	1.37%	1.44%
Weighted Average Yield %	2.92%	3.60%
Remaining Duration (years)	3 years 1 month 27 days	3 years 3 months 14 days
CMBS
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 2,221,936	$ 1,972,492
Amortized Cost Basis	2,248,289	1,994,928
Gross Unrealized Gains	54,486	4,643
Gross Unrealized Losses	(495)	(8,065)
Carrying Value	$ 2,302,280	$ 1,991,506
Number of Securities | securities	140	119
Weighted Average Coupon %	3.24%	3.17%
Weighted Average Yield %	2.82%	2.59%
Remaining Duration (years)	3 years 3 days	3 years 1 month 24 days
CMBS interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 7,676,007	$ 7,436,379
Amortized Cost Basis	333,958	348,222
Gross Unrealized Gains	1,980	1,027
Gross Unrealized Losses	(1,325)	(4,826)
Carrying Value	$ 334,613	$ 344,423
Number of Securities | securities	55	48
Weighted Average Coupon %	0.86%	1.02%
Weighted Average Yield %	3.47%	3.81%
Remaining Duration (years)	3 years 2 months 15 days	3 years 4 months 2 days
GNMA interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 538,877	$ 632,175
Amortized Cost Basis	22,873	28,311
Gross Unrealized Gains	79	44
Gross Unrealized Losses	(2,119)	(2,161)
Carrying Value	$ 20,833	$ 26,194
Number of Securities | securities	19	20
Weighted Average Coupon %	0.76%	0.80%
Weighted Average Yield %	3.71%	4.26%
Remaining Duration (years)	4 years 8 months 1 day	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
GNMA permanent securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 40,787	$ 16,249
Amortized Cost Basis	41,673	16,685
Gross Unrealized Gains	1,195	164
Gross Unrealized Losses	(384)	(394)
Carrying Value	$ 42,484	$ 16,455
Number of Securities | securities	12	12
Weighted Average Coupon %	4.17%	4.52%
Weighted Average Yield %	3.84%	3.94%
Remaining Duration (years)	10 years 29 days	5 years 5 months 4 days