FAIR VALUE MEASUREMENTS - Securities Valuation Methodology and Results (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($) source	Dec. 31, 2020 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Face Amount	$ 24,314,300	$ 31,869,682
Amortized Cost Basis	9,550,336	14,105,251
Real estate and other securities	$ 9,396,539	14,244,558
Number of broker quotation sources | source	2
Percentage of instruments with ranges of assumptions used available	99.30%
Agency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Face Amount	$ 8,399,343	12,491,152
Amortized Cost Basis	8,663,693	12,951,608
Non-Agency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Face Amount	15,914,957	19,378,530
Amortized Cost Basis	886,643	1,153,643
Fair Value	$ 945,168
Non-Agency | Discount Rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non-Agency RMBS, Measurement Input	0.010
Non-Agency | Discount Rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non-Agency RMBS, Measurement Input	0.150
Non-Agency | Discount Rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non-Agency RMBS, Measurement Input	0.058
Non-Agency | Prepayment Rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non-Agency RMBS, Measurement Input	0
Non-Agency | Prepayment Rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non-Agency RMBS, Measurement Input	0.250
Non-Agency | Prepayment Rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non-Agency RMBS, Measurement Input	0.103
Non-Agency | CDR | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non-Agency RMBS, Measurement Input	0
Non-Agency | CDR | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non-Agency RMBS, Measurement Input	0.120
Non-Agency | CDR | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non-Agency RMBS, Measurement Input	0.009
Non-Agency | Loss Severity | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non-Agency RMBS, Measurement Input	0
Non-Agency | Loss Severity | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non-Agency RMBS, Measurement Input	1.000
Non-Agency | Loss Severity | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non-Agency RMBS, Measurement Input	0.139
Multiple Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	$ 9,396,539	14,234,843
Single Quote
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	0	9,715
Level 2 | Agency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	8,444,597	13,063,634
Level 2 | Multiple Quotes | Agency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	8,444,597	13,063,634
Level 2 | Single Quote | Agency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	0	0
Level 3 | Non-Agency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	951,942	1,180,924
Level 3 | Multiple Quotes | Non-Agency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	951,942	1,171,209
Level 3 | Single Quote | Non-Agency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	$ 0	$ 9,715