INVESTMENTS IN RESIDENTIAL MORTGAGE LOANS - Performing Loans Past Due (Details) - Performing Loans - Residential Portfolio Segment	Sep. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent past due	100.00%
Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent past due	83.90%
30-59
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent past due	7.00%
60-89
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent past due	2.20%
90-119
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent past due	1.10%
120 or greater
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent past due	5.80%