Loans Held for Investment - Year Originated (Detail) - Loans Held for Investment at Fair Value - Seasoned Subprime Residential Mortgage Loans	Jun. 30, 2018	Dec. 31, 2017
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
2002 and prior	7.50%	7.50%
2003	6.70%	6.70%
2004	14.40%	14.60%
2005	20.10%	20.40%
2006	23.30%	23.10%
2007	19.10%	19.00%
2008	6.00%	6.00%
2009	0.80%	0.70%
2010 and later	2.10%	2.00%
Total	100.00%	100.00%