REAL ESTATE SECURITIES - Securities by Remaining Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Within 1 year	$ 73,796	$ 143,918
1-5 years	884,296	1,508,191
5-10 years	140,365	441,038
After 10 years	14	7,800
Fair value of real estate securities	1,098,471	2,100,947
CMBS
Schedule of Available-for-sale Securities [Line Items]
Within 1 year	72,958	132,730
1-5 years	770,559	1,156,026
5-10 years	106,512	412,639
After 10 years	0	0
Fair value of real estate securities	950,029	1,701,395
CMBS interest-only
Schedule of Available-for-sale Securities [Line Items]
Within 1 year	737	11,188
1-5 years	103,746	330,983
5-10 years	0	0
After 10 years	0	0
Fair value of real estate securities	104,483	342,171
GNMA interest-only
Schedule of Available-for-sale Securities [Line Items]
Within 1 year	101	0
1-5 years	7,368	15,914
5-10 years	516	724
After 10 years	14	183
Fair value of real estate securities	7,999	16,821
Agency securities
Schedule of Available-for-sale Securities [Line Items]
Within 1 year	0	0
1-5 years	742	780
5-10 years	0	0
After 10 years	0	0
Fair value of real estate securities	742	780
GNMA permanent securities
Schedule of Available-for-sale Securities [Line Items]
Within 1 year	0	0
1-5 years	1,881	4,488
5-10 years	33,337	27,675
After 10 years	0	7,617
Fair value of real estate securities	$ 35,218	$ 39,780