INVESTMENTS IN REAL ESTATE AND OTHER SECURITIES - Summary of the Geographic Distribution of the Collateral Securing Non-Agency RMBS (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 12,727,667	$ 7,202,218
Percentage of Total Outstanding	100.00%	100.00%
Non-Agency | Western U.S.
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 4,882,136	$ 2,757,424
Percentage of Total Outstanding	38.40%	38.30%
Non-Agency | Southeastern U.S.
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 3,005,519	$ 1,635,596
Percentage of Total Outstanding	23.60%	22.70%
Non-Agency | Northeastern U.S.
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 2,555,514	$ 1,426,519
Percentage of Total Outstanding	20.10%	19.80%
Non-Agency | Midwestern U.S.
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 1,337,980	$ 778,372
Percentage of Total Outstanding	10.50%	10.80%
Non-Agency | Southwestern U.S.
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 927,647	$ 557,033
Percentage of Total Outstanding	7.30%	7.70%
Non-Agency | Other U.S.
Schedule of Available-for-sale Securities [Line Items]
Outstanding Face Amount	$ 18,871	$ 47,274
Percentage of Total Outstanding	0.10%	0.70%
Consumer loan bonds | Bonds
Schedule of Available-for-sale Securities [Line Items]
Face Amount	$ 29,700
Servicer Advance Bonds | Bonds
Schedule of Available-for-sale Securities [Line Items]
Face Amount		$ 100,000