Residential Mortgage Loans Held in Securitization Trusts, Net and Real Estate Owned - Delinquencies in Portfolio of Residential Mortgage Loans Held in Securitization Trusts (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) loan	Dec. 31, 2017 USD ($) loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Real estate owned through foreclosure, Number of Delinquent Loans | loan	1	1
Real estate owned through foreclosure, Total Unpaid Principal | $	$ 118	$ 118
Real estate owned through foreclosure, % of Loan Portfolio	0.17%	0.15%
30 - 60
Financing Receivable, Recorded Investment, Past Due [Line Items]
Number of Delinquent Loans | loan	1	1
Total Unpaid Principal | $	$ 311	$ 203
% of Loan Portfolio	0.45%	0.26%
61 - 90
Financing Receivable, Recorded Investment, Past Due [Line Items]
Number of Delinquent Loans | loan	1	1
Total Unpaid Principal | $	$ 170	$ 173
% of Loan Portfolio	0.24%	0.22%
90
Financing Receivable, Recorded Investment, Past Due [Line Items]
Number of Delinquent Loans | loan	22	24
Total Unpaid Principal | $	$ 14,481	$ 16,147
% of Loan Portfolio	20.82%	20.80%