LOANS - Summary of Residential Mortgage Loans (Details) - Residential mortgage loans $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) point	Dec. 31, 2017 USD ($) point
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 1,976,077	$ 1,419,807
Minimum
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 1	$ 1
Interest rate	2.00%	1.63%
FICO score at loan origination | point	510	468
Loan-to-value ratio at loan origination	11.00%	11.00%
Maximum
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 3,500	$ 3,663
Interest rate	7.50%	7.50%
FICO score at loan origination | point	823	823
Loan-to-value ratio at loan origination	100.00%	100.00%
Portfolio Weighted Average
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 494	$ 514
Interest rate	4.74%	4.25%
FICO score at loan origination | point	754	748
Loan-to-value ratio at loan origination	67.00%	68.00%