Loans Held for Investment - Narrative (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Securitized debt loans held for investment at cost	$ 12,900	$ 13,300
Fair Value | Seasoned Subprime Residential Mortgage Loans
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
90 and over days delinquent	549	577
Loans Held for Investment at Fair Value | Seasoned Subprime Residential Mortgage Loans
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
REO	$ 16	$ 10