FAIR VALUE MEASUREMENT - Securities Valuation Methodology and Results (Details)	3 Months Ended
	Mar. 31, 2021 USD ($) source	Dec. 31, 2020 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Face Amount	$ 31,488,837,000
Amortized Cost Basis	14,933,463,000
Real estate and other securities	$ 14,606,157,000	$ 14,244,558,000
Number of broker quotation sources | source	2
Percent of securities	95.20%
Multiple Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	$ 14,606,157,000
Single Quote
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	0
Agency RMBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Face Amount	13,443,593,000
Amortized Cost Basis	13,956,269,000
Real estate and other securities	13,558,231,000	13,063,634,000
Agency RMBS | Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	13,558,231,000
Agency RMBS | Level 2 | Multiple Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	13,558,231,000
Agency RMBS | 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	18,045,244,000
Amortized Cost Basis	977,194,000
Real estate and other securities	1,047,926,000	$ 1,180,924,000
Fair Value	997,828,000
Non-Agency RMBS | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	1,047,926,000
Non-Agency RMBS | Level 3 | Multiple Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	1,047,926,000
Non-Agency RMBS | Level 3 | Single Quote
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Real estate and other securities	$ 0
Minimum | Non-Agency RMBS | Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.035
Minimum | Non-Agency RMBS | Prepayment Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.070
Minimum | Non-Agency RMBS | Delinquency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.005
Minimum | Non-Agency RMBS | Loss Severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.200
Maximum | Non-Agency RMBS | Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.150
Maximum | Non-Agency RMBS | Prepayment Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.250
Maximum | Non-Agency RMBS | Delinquency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.120
Maximum | Non-Agency RMBS | Loss Severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.880
Weighted Average | Non-Agency RMBS | Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.044
Weighted Average | Non-Agency RMBS | Prepayment Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.140
Weighted Average | Non-Agency RMBS | Delinquency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.014
Weighted Average | Non-Agency RMBS | Loss Severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Non agency RMBS, measurement input	0.295