REAL ESTATE SECURITIES - Summary of Securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 securities	Dec. 31, 2013 securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 10,950,979,000	$ 9,052,718,000
Amortized Cost Basis	2,786,317,000	1,648,124,000
Gross Unrealized Gains	33,222,000	20,463,000
Gross Unrealized Losses	(3,973,000)	(11,341,000)
Fair value of real estate securities	2,815,565,541	1,657,246,194
CMBS
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	2,247,565,000	1,160,741,000
Amortized Cost Basis	2,277,995,000	1,156,230,000
Gross Unrealized Gains	28,453,000	13,853,000
Gross Unrealized Losses	(1,038,000)	(5,147,000)
Fair value of real estate securities	2,305,410,000	1,164,936,000
# of Securities	145	101
Weighted Average Coupon %	3.31%	4.24%
Weighted Average Yield %	2.60%	4.08%
Remaining Duration	4 years 2 months 23 days	4 years 10 months 17 days
CMBS interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	7,239,503,000	5,702,862,000
Amortized Cost Basis	376,085,000	256,869,000
Gross Unrealized Gains	2,973,000	2,204,000
Gross Unrealized Losses	(723,000)	(1,015,000)
Fair value of real estate securities	378,335,000	258,058,000
# of Securities	41	21
Weighted Average Coupon %	1.04%	1.00%
Weighted Average Yield %	4.88%	4.19%
Remaining Duration	3 years 5 months 12 days	3 years 4 months 17 days
GNMA interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	1,400,141,000	1,848,270,000
Amortized Cost Basis	67,544,000	103,136,000
Gross Unrealized Gains	1,035,000	1,630,000
Gross Unrealized Losses	(1,937,000)	(4,889,000)
Fair value of real estate securities	66,642,000	99,877,000
# of Securities	34	36
Weighted Average Coupon %	0.85%	1.12%
Weighted Average Yield %	5.90%	5.32%
Remaining Duration	4 years 6 months	2 years 1 month 13 days
FHLMC interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount		219,677,000
Amortized Cost Basis		7,904,000
Gross Unrealized Gains		248,000
Gross Unrealized Losses		0
Fair value of real estate securities		8,152,000
# of Securities		2
Weighted Average Coupon %		0.95%
Weighted Average Yield %		5.21%
Remaining Duration		3 years 14 days
GN construction securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	27,538,000	12,858,000
Amortized Cost Basis	28,178,000	13,261,000
Gross Unrealized Gains	503,000	36,000
Gross Unrealized Losses	(275,000)	(290,000)
Fair value of real estate securities	28,406,000	13,007,000
# of Securities	4	8
Weighted Average Coupon %	3.89%	4.11%
Weighted Average Yield %	3.56%	3.49%
Remaining Duration	9 years 5 months 1 day	6 years 6 months 25 days
GN permanent securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	36,232,000	108,310,000
Amortized Cost Basis	36,515,000	110,724,000
Gross Unrealized Gains	258,000	2,492,000
Gross Unrealized Losses	0	0
Fair value of real estate securities	$ 36,773,000	$ 113,216,000
# of Securities	11	14
Weighted Average Coupon %	5.49%	5.53%
Weighted Average Yield %	4.94%	4.64%
Remaining Duration	1 year 3 months 25 days	3 years 3 months 7 days