Loans Held for Investment - Narrative (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Securitized debt loans held for investment at cost	$ 13,300,000	$ 8,600,000
90 and over days delinquent	752,967	548,843
Fair Value | Seasoned Subprime Residential Mortgage Loans
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
90 and over days delinquent	577,000	449,000
Loans Held for Investment at Fair Value | Seasoned Subprime Residential Mortgage Loans
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Foreclosure	200,000	203,000
Bankruptcy	273,000	207,000
REO	$ 31,000	$ 41,000