RESIDENTIAL MORTGAGE LOANS - Geographic Concentrations Based on Unpaid Principal Balances (Details) - Residential Mortgage Loans - Geographic Concentration Risk	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	100.00%	100.00%
California
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	49.80%	46.30%
Texas
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans		9.60%
New York
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	7.10%
Illinois
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans		5.70%
Florida
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	9.30%	5.20%
Washington
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans		5.10%
All other (none individually greater than 5%)
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	33.80%	28.10%