REAL ESTATE SECURITIES - Summary of Securities (Details) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		$ 4,001,570	$ 3,857,293
Amortized Cost Basis/Purchase Price		1,761,857	1,404,025
Gross Unrealized Gains		14,233	3,418
Gross Unrealized Losses		(879)	(8,867)
Carrying Value		$ 1,775,211	$ 1,398,576
Number of Securities | security		185	179
Weighted Average Coupon		1.78%	1.54%
Weighted Average Yield		3.34%	3.19%
Remaining Duration		2 years 3 months 14 days	2 years 4 months 24 days
Number of equity securities | security		3	3
Amortized Cost Basis/Purchase Price		$ 1,775,577	$ 1,417,179
Gross Unrealized Gains		14,296	3,418
Gross Unrealized Losses		(1,458)	(10,471)
Carrying Value	[1]	$ 1,788,415	$ 1,410,126
Total number of Securities | security		188	182
CMBS
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		$ 1,642,160	$ 1,258,819
Amortized Cost Basis/Purchase Price		1,643,149	1,257,801
Gross Unrealized Gains		11,108	2,477
Gross Unrealized Losses		(554)	(7,638)
Carrying Value		$ 1,653,703	$ 1,252,640
Number of Securities | security		145	138
Weighted Average Coupon		3.47%	3.32%
Weighted Average Yield		3.31%	3.14%
Remaining Duration		2 years 3 months 3 days	2 years 3 months 29 days
Risk retention requirement, amount		$ 11,600	$ 11,300
CMBS interest-only
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		2,166,640	2,373,936
Amortized Cost Basis/Purchase Price		42,689	55,534
Gross Unrealized Gains		1,370	428
Gross Unrealized Losses		(7)	(271)
Carrying Value		$ 44,052	$ 55,691
Number of Securities | security		18	19
Weighted Average Coupon		0.50%	0.57%
Weighted Average Yield		3.63%	2.80%
Remaining Duration		2 years 8 months 1 day	2 years 8 months 8 days
Risk retention requirement, amount		$ 900	$ 900
GNMA interest-only
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		118,858	135,932
Amortized Cost Basis/Purchase Price		2,483	2,862
Gross Unrealized Gains		124	93
Gross Unrealized Losses		(316)	(307)
Carrying Value		$ 2,291	$ 2,648
Number of Securities | security		12	12
Weighted Average Coupon		0.52%	0.51%
Weighted Average Yield		9.13%	6.30%
Remaining Duration		2 years 8 months 19 days	4 years 1 month 9 days
Agency securities
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		$ 652	$ 668
Amortized Cost Basis/Purchase Price		664	682
Gross Unrealized Gains		1	0
Gross Unrealized Losses		(2)	(20)
Carrying Value		$ 663	$ 662
Number of Securities | security		2	2
Weighted Average Coupon		2.70%	2.73%
Weighted Average Yield		1.78%	1.83%
Remaining Duration		2 years 1 month 9 days	2 years 4 months 9 days
GNMA permanent securities
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		$ 32,055	$ 32,633
Amortized Cost Basis/Purchase Price		32,291	32,889
Gross Unrealized Gains		793	420
Gross Unrealized Losses		0	(245)
Carrying Value		$ 33,084	$ 33,064
Number of Securities | security		6	6
Weighted Average Coupon		3.93%	3.94%
Weighted Average Yield		3.74%	3.76%
Remaining Duration		4 years 8 months 19 days	5 years 10 days
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		$ 41,205	$ 55,305
Amortized Cost Basis/Purchase Price		40,581	54,257
Gross Unrealized Gains		837	0
Gross Unrealized Losses		0	(386)
Carrying Value		$ 41,418	$ 53,871
Number of Securities | security		2	2
Weighted Average Coupon		3.80%	4.08%
Weighted Average Yield		4.82%	5.04%
Remaining Duration		1 year 7 months 20 days	2 years 6 months 3 days
Common Stock
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis/Purchase Price		$ 13,720	$ 13,154
Gross Unrealized Gains		63	0
Gross Unrealized Losses		(579)	(1,604)
Carrying Value		$ 13,204	$ 11,550

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