REAL ESTATE SECURITIES - Summary of Securities (Details) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		$ 4,096,400	$ 3,857,293
Amortized Cost Basis/Purchase Price		1,886,636	1,404,025
Gross Unrealized Gains		12,203	3,418
Gross Unrealized Losses		(840)	(8,867)
Carrying Value		$ 1,897,999	$ 1,398,576
Number of Securities | security		178	179
Weighted Average Coupon		1.75%	1.54%
Weighted Average Yield		3.17%	3.19%
Remaining Duration		2 years 4 months 20 days	2 years 4 months 24 days
Number of equity securities | security		3	3
Amortized Cost Basis/Purchase Price		$ 1,900,356	$ 1,417,179
Gross Unrealized Gains		12,328	3,418
Gross Unrealized Losses		(1,228)	(10,471)
Carrying Value	[1]	$ 1,911,456	$ 1,410,126
Total number of Securities | security		181	182
CMBS
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		$ 1,779,458	$ 1,258,819
Amortized Cost Basis/Purchase Price		1,780,233	1,257,801
Gross Unrealized Gains		9,012	2,477
Gross Unrealized Losses		(533)	(7,638)
Carrying Value		$ 1,788,712	$ 1,252,640
Number of Securities | security		139	138
Weighted Average Coupon		3.27%	3.32%
Weighted Average Yield		3.14%	3.14%
Remaining Duration		2 years 4 months 17 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		2,139,357	2,373,936
Amortized Cost Basis/Purchase Price		39,961	55,534
Gross Unrealized Gains		1,491	428
Gross Unrealized Losses		(12)	(271)
Carrying Value		$ 41,440	$ 55,691
Number of Securities | security		18	19
Weighted Average Coupon		0.49%	0.57%
Weighted Average Yield		3.65%	2.80%
Remaining Duration		2 years 7 months 9 days	2 years 8 months 8 days
Risk retention requirement, amount		$ 800	$ 900
GNMA interest-only
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		113,096	135,932
Amortized Cost Basis/Purchase Price		2,202	2,862
Gross Unrealized Gains		119	93
Gross Unrealized Losses		(295)	(307)
Carrying Value		$ 2,026	$ 2,648
Number of Securities | security		12	12
Weighted Average Coupon		0.51%	0.51%
Weighted Average Yield		9.65%	6.30%
Remaining Duration		2 years 8 months 23 days	4 years 1 month 9 days
Agency securities
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		$ 641	$ 668
Amortized Cost Basis/Purchase Price		652	682
Gross Unrealized Gains		2	0
Gross Unrealized Losses		0	(20)
Carrying Value		$ 654	$ 662
Number of Securities | security		2	2
Weighted Average Coupon		2.67%	2.73%
Weighted Average Yield		1.74%	1.83%
Remaining Duration		1 year 11 months 19 days	2 years 4 months 9 days
GNMA permanent securities
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		$ 31,760	$ 32,633
Amortized Cost Basis/Purchase Price		31,984	32,889
Gross Unrealized Gains		811	420
Gross Unrealized Losses		0	(245)
Carrying Value		$ 32,795	$ 33,064
Number of Securities | security		6	6
Weighted Average Coupon		3.92%	3.94%
Weighted Average Yield		3.27%	3.76%
Remaining Duration		4 years 6 months 18 days	5 years 10 days
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount		$ 32,088	$ 55,305
Amortized Cost Basis/Purchase Price		31,604	54,257
Gross Unrealized Gains		768	0
Gross Unrealized Losses		0	(386)
Carrying Value		$ 32,372	$ 53,871
Number of Securities | security		1	2
Weighted Average Coupon		3.63%	4.08%
Weighted Average Yield		4.81%	5.04%
Remaining Duration		1 year 3 months 21 days	2 years 6 months 3 days
Equity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis/Purchase Price		$ 13,720	$ 13,154
Gross Unrealized Gains		125	0
Gross Unrealized Losses		(388)	(1,604)
Carrying Value		$ 13,457	$ 11,550

[1] Includes amounts relating to consolidated variable interest entities. See Note 10.