Components for Mortgage Loan Held for Investment Portfolio (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) Point	Dec. 31, 2015 USD ($) Point
Schedule Of Available For Sale Securities [Line Items]
Maturity	2045-11
Mortgage Loans Held for Investment in Securitization Trusts, at Fair Value
Schedule Of Available For Sale Securities [Line Items]
Unpaid principal balance	$ 204,766,000	$ 223,205,000
Minimum
Schedule Of Available For Sale Securities [Line Items]
Unpaid principal balance	$ 110,000	$ 116,000
Interest rate	2.63%	2.63%
Maturity	2044-11	2044-11
FICO score at loan origination | Point	700	701
Loan-to-value ratio at loan origination	20.00%	20.00%
Minimum | Mortgage Loans Held for Investment in Securitization Trusts, at Fair Value
Schedule Of Available For Sale Securities [Line Items]
Unpaid principal balance	$ 220,000	$ 291,000
Interest rate	2.50%	2.50%
Maturity	2044-06	2044-06
FICO score at loan origination | Point	700	700
Loan-to-value ratio at loan origination	24.00%	24.00%
Maximum
Schedule Of Available For Sale Securities [Line Items]
Unpaid principal balance	$ 1,980,000	$ 1,987,000
Interest rate	4.00%	4.00%
Maturity	2046-04	2046-02
FICO score at loan origination | Point	811	811
Loan-to-value ratio at loan origination	80.00%	80.00%
Maximum | Mortgage Loans Held for Investment in Securitization Trusts, at Fair Value
Schedule Of Available For Sale Securities [Line Items]
Unpaid principal balance	$ 1,926,000	$ 1,933,000
Interest rate	4.13%	4.13%
Maturity	2045-11	2045-11
FICO score at loan origination | Point	813	815
Loan-to-value ratio at loan origination	80.00%	80.00%
Weighted Average
Schedule Of Available For Sale Securities [Line Items]
Unpaid principal balance	$ 778,000	$ 804,000
Interest rate	3.50%	3.48%
Maturity	2045-11	2045-10
FICO score at loan origination | Point	762	763
Loan-to-value ratio at loan origination	71.00%	71.00%
Weighted Average | Mortgage Loans Held for Investment in Securitization Trusts, at Fair Value
Schedule Of Available For Sale Securities [Line Items]
Unpaid principal balance	$ 731,000	$ 742,000
Interest rate	3.38%	3.38%
Maturity	2045-03	2045-03
FICO score at loan origination | Point	769	769
Loan-to-value ratio at loan origination	69.00%	69.00%