LOANS - Geographic Concentrations Based on Unpaid Principal Balances (Details) - Residential mortgage loans - Geographic Concentration Risk	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
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	54.20%	53.70%
New York
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	6.10%	6.60%
Florida
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	6.70%	7.10%
Other
Mortgage Loans on Real Estate [Line Items]
Geographic Concentrations of Residential Mortgage Loans	33.00%	32.60%