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, 2018	Dec. 31, 2017
Non-Agency
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 19,397,604	$ 12,727,667
Percentage of Total Outstanding	100.00%	100.00%
Non-Agency | Western U.S.
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 7,318,616	$ 4,882,136
Percentage of Total Outstanding	37.70%	38.40%
Non-Agency | Southeastern U.S.
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 4,613,314	$ 3,005,519
Percentage of Total Outstanding	23.80%	23.60%
Non-Agency | Northeastern U.S.
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 3,829,725	$ 2,555,514
Percentage of Total Outstanding	19.70%	20.10%
Non-Agency | Midwestern U.S.
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 2,063,263	$ 1,337,980
Percentage of Total Outstanding	10.60%	10.50%
Non-Agency | Southwestern U.S.
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 1,321,853	$ 927,647
Percentage of Total Outstanding	6.80%	7.30%
Non-Agency | Other U.S.
Debt Securities, Available-for-sale [Line Items]
Outstanding Face Amount	$ 250,833	$ 18,871
Percentage of Total Outstanding	1.40%	0.10%
Consumer loan bonds | Bonds
Debt Securities, Available-for-sale [Line Items]
Face Amount	$ 56,800	$ 29,700
Corporate bond | Bonds
Debt Securities, Available-for-sale [Line Items]
Face Amount	$ 85,000	$ 0