REAL ESTATE SECURITIES - Summary of Securities (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) securities		Dec. 31, 2017 USD ($) securities	Jun. 29, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 3,675,300		$ 4,292,845
Amortized Cost Basis	1,109,622		1,107,380
Gross Unrealized Gains	1,133		4,296
Gross Unrealized Losses	(10,650)		(5,159)
Carrying Value	$ 1,100,105	[1]	$ 1,106,517
Number of Securities | securities	140		142
Weighted Average Coupon %	1.54%		1.37%
Weighted Average Yield %	2.96%		2.87%
Remaining Duration (years)	3 years 7 days		3 years
Risk retention requirement, amount				$ 12,900
CMBS
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 983,104		$ 945,167
Amortized Cost Basis	991,436		954,397
Gross Unrealized Gains	331		2,748
Gross Unrealized Losses	(9,483)		(3,646)
Carrying Value	$ 982,284		$ 953,499
Number of Securities | securities	98		96
Weighted Average Coupon %	3.34%		3.28%
Weighted Average Yield %	2.89%		2.79%
Remaining Duration (years)	2 years 11 months 4 days		2 years 10 months 20 days
Risk retention requirement, amount	$ 11,500		$ 11,700
CMBS interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	2,492,711		3,140,297
Amortized Cost Basis	79,162		112,609
Gross Unrealized Gains	192		796
Gross Unrealized Losses	(319)		(334)
Carrying Value	$ 79,035		$ 113,071
Number of Securities | securities	21		25
Weighted Average Coupon %	0.87%		0.81%
Weighted Average Yield %	3.46%		3.16%
Remaining Duration (years)	2 years 11 months 26 days		3 years 29 days
Risk retention requirement, amount	$ 1,000		$ 1,100
GNMA interest-only
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	165,307		172,916
Amortized Cost Basis	4,540		5,245
Gross Unrealized Gains	142		157
Gross Unrealized Losses	(707)		(925)
Carrying Value	$ 3,975		$ 4,477
Number of Securities | securities	13		13
Weighted Average Coupon %	0.57%		0.58%
Weighted Average Yield %	6.60%		6.70%
Remaining Duration (years)	4 years 14 days		4 years 2 months 4 days
Agency securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 706		$ 720
Amortized Cost Basis	727		743
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(29)		(15)
Carrying Value	$ 698		$ 728
Number of Securities | securities	2		2
Weighted Average Coupon %	2.80%		2.82%
Weighted Average Yield %	1.77%		1.80%
Remaining Duration (years)	2 years 9 months 10 days		2 years 11 months 8 days
GNMA permanent securities
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 33,472		$ 33,745
Amortized Cost Basis	33,757		34,386
Gross Unrealized Gains	468		595
Gross Unrealized Losses	(112)		(239)
Carrying Value	$ 34,113		$ 34,742
Number of Securities | securities	6		6
Weighted Average Coupon %	3.97%		3.98%
Weighted Average Yield %	3.62%		3.62%
Remaining Duration (years)	5 years 6 months		5 years 7 months 27 days

[1] Includes amounts relating to consolidated variable interest entities. See Note 3.