FAIR VALUE MEASUREMENTS - Securities Valuation Methodology and Results (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2022 USD ($) source	Dec. 31, 2021 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Face Amount	$ 26,160,486
Amortized Cost Basis	10,281,037
Real estate and other securities	$ 9,509,930	$ 9,396,539
Number of broker quotation sources | source	2
Percent of securities	65.30%
Multiple Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	$ 9,509,930
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	9,078,723
Amortized Cost Basis	9,338,295
RMBS Designated as AFS | Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	8,532,760
RMBS Designated as AFS | Level 2 | Multiple Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	8,532,760
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	17,081,763
Amortized Cost Basis	942,742
Fair Value	638,458
Non-Agency RMBS | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	977,170
Non-Agency RMBS | Level 3 | Multiple Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	977,170
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	0.029
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	0.880
Non-Agency RMBS | Weighted Average | Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.053
Non-Agency RMBS | Weighted Average | Prepayment Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.156
Non-Agency RMBS | Weighted Average | Delinquency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.014
Non-Agency RMBS | Weighted Average | Loss Severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.255