FAIR VALUE MEASUREMENT - Inputs used in Valuing the Servicer Advances (Details) - Servicer Advances	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Outstanding Servicer Advances to UPB of Underlying Residential Mortgage Loans	1.40%	1.70%
Mortgage Servicing Amount (bps)	0.00196	0.00182
Cost to service amount	0.096%	0.125%
Prepayment Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing Asset, Measurement Input	0.109	0.100
Delinquency
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing Asset, Measurement Input	0.177	0.138
Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Servicing Asset, Measurement Input	0.059	0.068
Collateral Weighted Average Maturity Years | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Collateral Weighted Average Maturity (in years)	23 years 4 months 8 days	25 years 7 months 17 days