INVESTMENTS IN RESIDENTIAL MORTGAGE LOANS - Allowance for Loan Losses (Details) - Performing Loans - Residential Portfolio Segment $ in Thousands	9 Months Ended
Sep. 30, 2018 USD ($)
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at December 31, 2017	$ 196
Provision for loan losses	604
Charge-offs	(800)
Balance at September 30, 2018	$ 0

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant