FAIR VALUE MEASUREMENT - Information Regarding the Inputs used in Valuing the Servicer Advances (Details) - Servicer Advances	6 Months Ended
	Jun. 30, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Mortgage servicing amount	10.70%
Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Servicer Advances to UPB of Underlying Residential Mortgage Loans	0.80%
Mortgage servicing amount	0.168
Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Servicer Advances to UPB of Underlying Residential Mortgage Loans	1.50%
Mortgage servicing amount	0.197
Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Servicer Advances to UPB of Underlying Residential Mortgage Loans	1.50%
Mortgage servicing amount	0.197
Prepayment Rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing asset, measurement input	0.086
Prepayment Rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing asset, measurement input	0.091
Prepayment Rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing asset, measurement input	0.091
Delinquency | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing asset, measurement input	0.074
Delinquency | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing asset, measurement input	0.090
Delinquency | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing asset, measurement input	0.090
Discount Rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing asset, measurement input	0.053
Discount Rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing asset, measurement input	0.058
Discount Rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing asset, measurement input	0.053
Collateral Weighted Average Maturity (Years) | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing asset, term	22 years 6 months
Collateral Weighted Average Maturity (Years) | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing asset, term	22 years 7 months 6 days
Collateral Weighted Average Maturity (Years) | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing asset, term	22 years 6 months