Loans Held for Investment - Collateral Characteristics (Detail) - Loans Held for Investment at Fair Value - Seasoned Subprime Residential Mortgage Loans $ in Thousands	Mar. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Number of loans | loan	137,605	139,194
Weighted average maturity (years)	19 years 6 months	19 years 3 months 18 days
Weighted average loan to value	86.50%	87.40%
Weighted average FICO	631	627
Weighted average loan balance (in thousands) | $	$ 97	$ 95
Weighted average percentage owner occupied	87.80%	88.00%
Weighted average percentage single family residence	83.80%	84.40%
CA
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top five states	12.40%	11.60%
FL
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top five states	7.70%	7.40%
NY
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top five states	7.10%	6.90%
OH
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top five states	5.10%	5.30%
PA
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top five states	5.10%	5.30%