RESIDENTIAL MORTGAGE LOANS - Additional Information about Residential Mortgage Loans (Details) - Residential Mortgage $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) point	Dec. 31, 2016 USD ($) point
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 1,419,807	$ 338,323
Minimum
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 1	$ 22
Interest rate	1.63%	2.50%
FICO score at loan origination | point	468	665
Loan-to-value ratio at loan origination	11.00%	24.00%
Maximum
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 3,663	$ 1,905
Interest rate	7.50%	6.75%
FICO score at loan origination | point	823	814
Loan-to-value ratio at loan origination	100.00%	90.00%
Weighted Average
Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 514	$ 691
Interest rate	4.25%	3.72%
FICO score at loan origination | point	748	761
Loan-to-value ratio at loan origination	68.00%	71.00%