LOANS - Geographic Concentrations Based on Unpaid Principal Balances (Details) - Residential mortgage loans - Geographic Concentration Risk	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
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	54.50%	49.80%
New York
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	8.10%	7.10%
Florida
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	6.60%	9.30%
All other (none individually greater than 5%)
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	30.80%	33.80%