REAL ESTATE SECURITIES - Summary of Securities (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) securities	Dec. 31, 2017 USD ($) securities	Jun. 29, 2017 USD ($)
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 3,643,431	$ 4,292,845
Amortized Cost Basis	1,118,012	1,107,380
Gross Unrealized Gains	1,210	4,296
Gross Unrealized Losses	(12,864)	(5,159)
Carrying Value	$ 1,106,358	$ 1,106,517
Number of Securities | securities	142	142
Weighted Average Coupon %	1.45%	1.37%
Weighted Average Yield %	2.98%	2.87%
Remaining Duration (years)	2 years 11 months 4 days	3 years
Risk retention requirement, amount			$ 12,900
CMBS
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 999,461	$ 945,167
Amortized Cost Basis	1,005,795	954,397
Gross Unrealized Gains	244	2,748
Gross Unrealized Losses	(11,742)	(3,646)
Carrying Value	$ 994,297	$ 953,499
Number of Securities | securities	100	96
Weighted Average Coupon %	3.33%	3.28%
Weighted Average Yield %	2.92%	2.79%
Remaining Duration (years)	2 years 10 months 6 days	2 years 10 months 20 days
Risk retention requirement, amount	$ 11,300	$ 11,700
CMBS interest-only
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	2,466,340	3,140,297
Amortized Cost Basis	74,383	112,609
Gross Unrealized Gains	437	796
Gross Unrealized Losses	(274)	(334)
Carrying Value	$ 74,546	$ 113,071
Number of Securities | securities	21	25
Weighted Average Coupon %	0.71%	0.81%
Weighted Average Yield %	3.25%	3.16%
Remaining Duration (years)	2 years 10 months 13 days	3 years 29 days
Risk retention requirement, amount	$ 1,000	$ 1,100
GNMA interest-only
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	143,738	172,916
Amortized Cost Basis	3,651	5,245
Gross Unrealized Gains	117	157
Gross Unrealized Losses	(572)	(925)
Carrying Value	$ 3,196	$ 4,477
Number of Securities | securities	13	13
Weighted Average Coupon %	0.52%	0.58%
Weighted Average Yield %	7.19%	6.70%
Remaining Duration (years)	4 years 1 month 6 days	4 years 2 months 4 days
Agency securities
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 696	$ 720
Amortized Cost Basis	712	743
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(26)	(15)
Carrying Value	$ 686	$ 728
Number of Securities | securities	2	2
Weighted Average Coupon %	2.78%	2.82%
Weighted Average Yield %	1.93%	1.80%
Remaining Duration (years)	2 years 7 months 20 days	2 years 11 months 8 days
GNMA permanent securities
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 33,196	$ 33,745
Amortized Cost Basis	33,471	34,386
Gross Unrealized Gains	412	595
Gross Unrealized Losses	(250)	(239)
Carrying Value	$ 33,633	$ 34,742
Number of Securities | securities	6	6
Weighted Average Coupon %	3.96%	3.98%
Weighted Average Yield %	3.77%	3.62%
Remaining Duration (years)	5 years 4 months 6 days	5 years 7 months 27 days