Loans Held for Investment - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Securitized debt loans held for investment at cost	$ 12,200,000,000	$ 13,300,000,000
90 and over days delinquent	795,504,000	752,967,000
Fair Value | Seasoned Subprime Residential Mortgage Loans
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
90 and over days delinquent	606,000,000	577,000,000
Loans Held for Investment at Fair Value
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Purchases	415,000,000
Retained balance	40,000,000
Realized gains (losses) on sales and settlements	0
Loans Held for Investment at Fair Value | Seasoned Subprime Residential Mortgage Loans
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Loans in foreclosure proceedings	292,000,000	200,000,000
Loans subject to bankruptcy protection	266,000,000	273,000,000
Real estate acquired, loan value	$ 47,000,000	$ 31,000,000