Loans and Allowance for Credit Losses - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) SecurityLoan MortgageLoan	Dec. 31, 2018 USD ($)
Loans and Allowance for Credit Losses [Line Items]
Loans pledged at the Federal Home Loan Bank	$ 91,000	$ 88,700
Loans pledged at the Federal Reserve Bank	71,100	70,100
Unearned interest and deferred fees and costs on originated loans	854	872
Foreclosed residential real estate property included in other real estate owned	88	106
Foreclosed residential real estate related to mortgage loans whose payments are insured by the Federal Housing Administration or guaranteed by the United States Department of Veterans Affairs	231	235
Residential mortgage loans secured by residential real estate in process of foreclosure	$ 1,600	1,500
Number of residential mortgage loans, home equity and second mortgage loans, and GNMA loans where trial period was unsuccessful and no longer eligible for a permanent modification | MortgageLoan	259
Outstanding balance of residential mortgage loans, home equity and second mortgage loans, and GNMA loans where trial period was unsuccessful and no longer eligible for a permanent modification	$ 33
Government National Mortgage Association [Member]
Loans and Allowance for Credit Losses [Line Items]
Residential mortgage loans secured by residential real estate in process of foreclosure purchased from GNMA mortgage pools whose repayments are insured by the Federal Housing Administration or guaranteed by the United States Department of Veterans Affairs	$ 1,300	$ 1,200
Home Equity and Second Mortgages [Member]
Loans and Allowance for Credit Losses [Line Items]
Number of loans in trial period | SecurityLoan	20
Outstanding balance of loans in trial period	$ 2
Estimated post-modification balance of loans in trial period	$ 2
Residential Mortgages [Member]
Loans and Allowance for Credit Losses [Line Items]
Number of loans in trial period | SecurityLoan	11
Outstanding balance of loans in trial period	$ 2
Estimated post-modification balance of loans in trial period	$ 2
Government National Mortgage Association [Member]
Loans and Allowance for Credit Losses [Line Items]
Number of loans in trial period | SecurityLoan	1,125
Outstanding balance of loans in trial period	$ 146
Estimated post-modification balance of loans in trial period	$ 143