Loans and Allowance for Credit Losses - Summary of Loans by Portfolio Class, Including Delinquency Status of those that Continue to Accrue Interest and are Nonperforming (Parenthetical) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Government National Mortgage Association [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 days past due purchased from Government National Mortgage Association mortgage pools whose payments are insured by the Federal Housing Administration or guaranteed by the United States Department of Veterans Affairs, were classified as current	$ 414	$ 385
Loans 90 days or more past due purchased from Government National Mortgage Association mortgage pools whose payments are insured by the Federal Housing Administration or guaranteed by the United States Department of Veterans Affairs, were classified as current	1,700	$ 1,900
Covered Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Amount of covered residential mortgage loans transferred to loans held for sale	1,300
Covered Loans [Member] | Financing Receivables 30 To 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Amount of covered residential mortgage loans transferred to loans held for sale	42
Covered Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Amount of covered residential mortgage loans transferred to loans held for sale	108
Covered Loans [Member] | Nonperforming Financial Instruments [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Amount of covered residential mortgage loans transferred to loans held for sale	$ 6