LOANS - Additional Information about Residential Mortgage Loans (Details) - Residential mortgage loans $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) point	Dec. 31, 2018 USD ($) point
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 3,475,799	$ 2,425,657
Minimum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 1	$ 0
Interest rate	2.00%	2.00%
FICO score at loan origination | point	505	505
Loan-to-value ratio at loan origination	8.00%	8.00%
Maximum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 3,448	$ 3,500
Interest rate	9.25%	7.75%
FICO score at loan origination | point	825	823
Loan-to-value ratio at loan origination	111.00%	111.00%
Weighted Average
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Unpaid principal balance	$ 463	$ 457
Interest rate	5.03%	4.72%
FICO score at loan origination | point	756	752
Loan-to-value ratio at loan origination	67.00%	68.00%