REAL ESTATE SECURITIES - Securities by Remaining Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Within 1 year	$ 286,595		$ 143,918
1-5 years	663,243		1,508,191
5-10 years	156,668		441,038
After 10 years	11		7,800
Fair value of real estate securities	1,106,517	[1]	2,100,947
CMBS
Schedule of Available-for-sale Securities [Line Items]
Within 1 year	285,982		132,730
1-5 years	544,278		1,156,026
5-10 years	123,239		412,639
After 10 years	0		0
Fair value of real estate securities	953,499		1,701,395
CMBS interest-only
Schedule of Available-for-sale Securities [Line Items]
Within 1 year	537		11,188
1-5 years	112,534		330,983
5-10 years	0		0
After 10 years	0		0
Fair value of real estate securities	113,071		342,171
GNMA interest-only
Schedule of Available-for-sale Securities [Line Items]
Within 1 year	76		0
1-5 years	3,906		15,914
5-10 years	484		724
After 10 years	11		183
Fair value of real estate securities	4,477		16,821
Agency securities
Schedule of Available-for-sale Securities [Line Items]
Within 1 year	0		0
1-5 years	728		780
5-10 years	0		0
After 10 years	0		0
Fair value of real estate securities	728		780
GNMA permanent securities
Schedule of Available-for-sale Securities [Line Items]
Within 1 year	0		0
1-5 years	1,797		4,488
5-10 years	32,945		27,675
After 10 years	0		7,617
Fair value of real estate securities	$ 34,742		$ 39,780

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