Mortgage-Backed Securities - Geographical and Other Statistics (Detail) - Non-agency MBS - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Weighted average maturity (years)	20 years 6 months	20 years 8 months 12 days
Weighted average amortized loan to value	64.30%	64.40%
Weighted average FICO	697	697
Weighted average loan balance (in thousands)	$ 316	$ 314
Weighted average percentage owner occupied	83.90%	84.40%
Weighted average percentage single family residence	65.60%	66.30%
Weighted average current credit enhancement	2.30%	2.20%
California
Investment Holdings [Line Items]
Weighted average geographic concentration of top four states	31.60%	31.70%
New York
Investment Holdings [Line Items]
Weighted average geographic concentration of top four states	8.80%	8.50%
Florida
Investment Holdings [Line Items]
Weighted average geographic concentration of top four states	8.40%	8.30%
New Jersey
Investment Holdings [Line Items]
Weighted average geographic concentration of top four states	2.70%	2.70%