Securitized Loans Held for Investment - Collateral Characteristics (Detail) - Securitized Loans Held for Investment at Fair Value - Seasoned Subprime Residential Mortgage Loans $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) point loan	Dec. 31, 2016 USD ($) point loan
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Number of loans | loan	140,899	95,155
Weighted average maturity (years)	19 years	19 years 9 months 18 days
Weighted average loan to value	87.30%	86.90%
Weighted average FICO | point	640	627
Weighted average loan balance (in thousands) | $	$ 90	$ 93
Weighted average percentage owner occupied	97.00%	96.60%
Weighted average percentage single family residence	86.00%	85.00%
California
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top four states	9.20%	10.00%
Florida
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top four states	6.80%	6.70%
Ohio
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top four states	6.40%	6.50%
Pennsylvania
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top four states	5.50%
Virginia
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top four states	5.50%	5.90%
North Carolina
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Weighted average geographic concentration of top four states		5.10%