Mortgage-Backed Securities - Geographical and Other Statistics (Detail) - Non-agency MBS $ in Thousands	Mar. 31, 2017 USD ($) point	Dec. 31, 2016 USD ($) point
Investment Holdings [Line Items]
Weighted average maturity (years)	21 years 4 months 24 days	21 years 7 months 6 days
Weighted average amortized loan to value	66.20%	66.50%
Weighted average FICO | point	674	675
Weighted average loan balance (in thousands) | $	$ 320	$ 319
Weighted average percentage owner occupied	83.30%	83.20%
Weighted average percentage single family residence	65.80%	65.80%
Weighted average current credit enhancement	2.30%	2.30%
California
Investment Holdings [Line Items]
Weighted average geographic concentration of top four states	32.10%	32.10%
Florida
Investment Holdings [Line Items]
Weighted average geographic concentration of top four states	8.20%	8.10%
New York
Investment Holdings [Line Items]
Weighted average geographic concentration of top four states	8.10%	7.90%
New Jersey
Investment Holdings [Line Items]
Weighted average geographic concentration of top four states	2.70%	2.70%