REAL ESTATE SECURITIES - Summary of Securities (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) securities	Dec. 31, 2015 USD ($) securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 9,997,135	$ 10,084,386
Amortized Cost Basis	2,572,340	2,415,727
Gross Unrealized Gains	35,537	6,936
Gross Unrealized Losses	(9,003)	(15,446)
Carrying Value	$ 2,598,874	$ 2,407,217
Number of Securities | securities	219	200
Weighted Average Coupon %	1.51%	1.44%
Weighted Average Yield %	3.00%	3.60%
Remaining Duration (years)	3 years 3 months	3 years 3 months 14 days
CMBS
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 2,139,819	$ 1,972,492
Amortized Cost Basis	2,160,286	1,994,928
Gross Unrealized Gains	33,162	4,643
Gross Unrealized Losses	(1,294)	(8,065)
Carrying Value	$ 2,192,154	$ 1,991,506
Number of Securities | securities	133	119
Weighted Average Coupon %	3.21%	3.17%
Weighted Average Yield %	2.86%	2.59%
Remaining Duration (years)	3 years 1 month 24 days	3 years 1 month 24 days
CMBS interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 7,216,114	$ 7,436,379
Amortized Cost Basis	344,809	348,222
Gross Unrealized Gains	952	1,027
Gross Unrealized Losses	(5,634)	(4,826)
Carrying Value	$ 340,127	$ 344,423
Number of Securities | securities	53	48
Weighted Average Coupon %	1.06%	1.02%
Weighted Average Yield %	3.76%	3.81%
Remaining Duration (years)	3 years 3 months 10 days	3 years 4 months 2 days
GNMA interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 600,054	$ 632,175
Amortized Cost Basis	25,194	28,311
Gross Unrealized Gains	233	44
Gross Unrealized Losses	(1,689)	(2,161)
Carrying Value	$ 23,738	$ 26,194
Number of Securities | securities	21	20
Weighted Average Coupon %	0.73%	0.80%
Weighted Average Yield %	4.28%	4.26%
Remaining Duration (years)	5 years 1 month 17 days	5 years 2 months 19 days
GN 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
GN permanent securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 41,148	$ 16,249
Amortized Cost Basis	42,051	16,685
Gross Unrealized Gains	1,190	164
Gross Unrealized Losses	(386)	(394)
Carrying Value	$ 42,855	$ 16,455
Number of Securities | securities	12	12
Weighted Average Coupon %	4.18%	4.52%
Weighted Average Yield %	3.49%	3.94%
Remaining Duration (years)	7 years 7 days	5 years 5 months 4 days