LOANS - Geographic Concentrations Based on Unpaid Principal Balances (Details) - Residential mortgage loans - Geographic Concentration Risk	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	100.00%	100.00%
California
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	51.90%	53.70%
New York
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	10.40%	6.60%
Florida
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	5.70%	7.10%
Other
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	32.00%	32.60%