REAL ESTATE SECURITIES - Summary of Securities (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) securities	Dec. 31, 2014 USD ($) securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 9,888,419	$ 10,950,979
Amortized Cost Basis	2,390,857	2,786,317
Gross Unrealized Gains	28,576	33,222
Gross Unrealized Losses	(4,051)	(3,973)
Carrying Value	$ 2,415,382	$ 2,815,566
Number of of Securities | securities	202	235
Weighted Average Coupon %	1.37%	1.50%
Weighted Average Yield %	3.56%	4.54%
Remaining Duration (years)	3 years 7 months 2 days	3 years 9 months
CMBS
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 1,936,316	$ 2,247,565
Amortized Cost Basis	1,960,371	2,277,995
Gross Unrealized Gains	25,897	28,453
Gross Unrealized Losses	(1,073)	(1,038)
Carrying Value	$ 1,985,195	$ 2,305,410
Number of of Securities | securities	120	145
Weighted Average Coupon %	3.25%	3.31%
Weighted Average Yield %	2.63%	2.60%
Remaining Duration (years)	3 years 4 months 6 days	4 years 2 months 23 days
CMBS interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 7,246,777	$ 7,239,503
Amortized Cost Basis	356,204	376,085
Gross Unrealized Gains	1,726	2,973
Gross Unrealized Losses	(1,002)	(723)
Carrying Value	$ 356,928	$ 378,335
Number of of Securities | securities	47	41
Weighted Average Coupon %	0.90%	1.04%
Weighted Average Yield %	3.74%	4.88%
Remaining Duration (years)	3 years 5 months 15 days	3 years 5 months 12 days
GNMA interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 662,161	$ 1,400,141
Amortized Cost Basis	30,109	67,544
Gross Unrealized Gains	77	1,035
Gross Unrealized Losses	(1,583)	(1,937)
Carrying Value	$ 28,603	$ 66,642
Number of of Securities | securities	21	34
Weighted Average Coupon %	0.81%	0.85%
Weighted Average Yield %	4.27%	5.90%
Remaining Duration (years)	5 years 5 months 23 days	4 years 6 months
GN construction securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 26,730	$ 27,538
Amortized Cost Basis	27,223	28,178
Gross Unrealized Gains	589	503
Gross Unrealized Losses	37	(275)
Carrying Value	$ 27,849	$ 28,406
Number of of Securities | securities	2	4
Weighted Average Coupon %	4.10%	3.89%
Weighted Average Yield %	3.86%	3.56%
Remaining Duration (years)	9 years 5 months 15 days	9 years 5 months 1 day
GN permanent securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 16,435	$ 36,232
Amortized Cost Basis	16,950	36,515
Gross Unrealized Gains	287	258
Gross Unrealized Losses	(430)	0
Carrying Value	$ 16,807	$ 36,773
Number of of Securities | securities	12	11
Weighted Average Coupon %	4.55%	5.49%
Weighted Average Yield %	4.00%	4.94%
Remaining Duration (years)	6 years 1 month 2 days	1 year 3 months 25 days