Loans and Allowance for Credit Losses - Additional Information (Detail) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) SecurityLoan MortgageLoan	Dec. 31, 2019 USD ($)
Loans and Allowance for Credit Losses [Line Items]
Loans pledged at the Federal Home Loan Bank	$ 96,500	$ 96,200
Loans pledged at the Federal Reserve Bank	75,500	76,300
Unearned interest and deferred fees and costs on originated loans	744	781
Foreclosed residential real estate property included in other real estate owned	67	74
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	122	155
Residential mortgage loans secured by residential real estate in process of foreclosure	$ 1,400	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	137
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	$ 19
Commitments to lend additional funds	119
Modifications related to borrowers impacted by COVID	3,100
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	$ 1,100	$ 1,200
Home Equity and Second Mortgages [Member]
Loans and Allowance for Credit Losses [Line Items]
Number of loans in trial period \| SecurityLoan	22
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	39
Outstanding balance of loans in trial period	$ 5
Estimated post-modification balance of loans in trial period	$ 5
Government National Mortgage Association [Member]
Loans and Allowance for Credit Losses [Line Items]
Number of loans in trial period \| SecurityLoan	1,024
Outstanding balance of loans in trial period	$ 141
Estimated post-modification balance of loans in trial period	$ 140

Amount of financing receivable modified on trial basis.

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