Components for Mortgage Loan Held for Investment Portfolio (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) Point	Dec. 31, 2014 USD ($) Point
Minimum
Schedule Of Available For Sale Securities [Line Items]
Interest rate	2.50%	2.75%
Maturity	2044-05	2044-06
FICO score at loan origination | Point	700	705
Loan-to-value ratio at loan origination	24.00%	28.00%
Maximum
Schedule Of Available For Sale Securities [Line Items]
Interest rate	4.13%	3.75%
Maturity	2045-10	2044-12
FICO score at loan origination | Point	815	813
Loan-to-value ratio at loan origination	80.00%	80.00%
Weighted Average
Schedule Of Available For Sale Securities [Line Items]
Interest rate	3.41%	3.43%
Maturity	2045-02	2044-09
FICO score at loan origination | Point	768	762
Loan-to-value ratio at loan origination	69.00%	65.00%
Mortgage Held for Investment
Schedule Of Available For Sale Securities [Line Items]
Unpaid principal balance	$ 259,165,000	$ 30,792,000
Mortgage Held for Investment | Minimum
Schedule Of Available For Sale Securities [Line Items]
Unpaid principal balance	8,000	447,000
Mortgage Held for Investment | Maximum
Schedule Of Available For Sale Securities [Line Items]
Unpaid principal balance	1,990,000	1,332,000
Mortgage Held for Investment | Weighted Average
Schedule Of Available For Sale Securities [Line Items]
Unpaid principal balance	$ 764,000	$ 790,000