INVESTMENTS IN RESIDENTIAL MORTGAGE LOANS - Valuation Provision on Reverse Mortgage Loans and Allowance for Loan Losses on Performing Loans (Details) - Residential Portfolio Segment - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reverse Mortgage Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 0	$ 1,553
Provision for loan losses	0	73
Charge-offs	0	0
Sales		(171)
Transfer of loans to held-for-sale		(1,455)
Ending balance	0	0
Performing Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0	119
Provision for loan losses	646	4
Charge-offs	(450)	0
Sales		0
Transfer of loans to held-for-sale		(123)
Ending balance	$ 196	$ 0