FAIR VALUE MEASUREMENTS - Securities Valuation Methodology and Results (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2021 USD ($) source	Dec. 31, 2020 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Face Amount	$ 30,726,412
Amortized Cost Basis	15,096,302
Real estate and other securities	$ 14,956,889	$ 14,244,558
Number of broker quotation sources | source	2
Percent of securities	94.70%
Multiple Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	$ 14,956,889
Single Quote
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	0
RMBS Designated as AFS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Face Amount	13,701,666
Amortized Cost Basis	14,158,189
RMBS Designated as AFS | Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	13,953,718
RMBS Designated as AFS | Level 2 | Multiple Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	13,953,718
RMBS Designated as AFS | Level 2 | Single Quote
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	0
Non-Agency RMBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Face Amount	17,024,746
Amortized Cost Basis	938,113
Fair Value	949,545
Non-Agency RMBS | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	1,003,171
Non-Agency RMBS | Level 3 | Multiple Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	1,003,171
Non-Agency RMBS | Level 3 | Single Quote
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	$ 0
Non-Agency RMBS | Minimum | Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.012
Non-Agency RMBS | Minimum | Prepayment Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.000
Non-Agency RMBS | Minimum | Delinquency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.000
Non-Agency RMBS | Minimum | Loss Severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.000
Non-Agency RMBS | Maximum | Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.150
Non-Agency RMBS | Maximum | Prepayment Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.250
Non-Agency RMBS | Maximum | Delinquency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.120
Non-Agency RMBS | Maximum | Loss Severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	1.000
Non-Agency RMBS | Weighted Average | Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.046
Non-Agency RMBS | Weighted Average | Prepayment Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.145
Non-Agency RMBS | Weighted Average | Delinquency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.012
Non-Agency RMBS | Weighted Average | Loss Severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.198