Loans Held for Investment - Collateral Characteristics (Detail) - Loans Held for Investment at Fair Value - Seasoned Subprime Residential Mortgage Loans $ in Thousands	Mar. 31, 2019 USD ($) loan	Dec. 31, 2018 USD ($) loan
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Number of loans | loan	136,791	140,345
Weighted average maturity (years)	18 years 3 months 18 days	18 years 7 months 6 days
Weighted average loan to value	87.40%	87.50%
Weighted average FICO	629	629
Weighted average loan balance (in thousands) | $	$ 89	$ 89
Weighted average percentage owner occupied	89.30%	90.10%
Weighted average percentage single family residence	86.10%	86.30%
CA
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top four states	9.50%	9.00%
FL
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top four states	7.20%	7.30%
OH
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top four states	5.90%	5.90%
NY
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top four states	5.80%	5.70%
PA
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top four states	5.60%	5.60%