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Loans and Allowance for Credit Losses - Additional Information (Detail)
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
MortgageLoan
Sep. 30, 2018
USD ($)
SecurityLoan
MortgageLoan
Dec. 31, 2017
USD ($)
Loans and Allowance for Credit Losses [Line Items]      
Loans pledged at the Federal Home Loan Bank $ 89,000,000,000 $ 89,000,000,000 $ 83,300,000,000
Loans pledged at the Federal Reserve Bank 69,200,000,000 69,200,000,000 68,000,000,000
Unearned interest and deferred fees and costs on originated loans 841,000,000 $ 841,000,000 830,000,000
Minimum period for non collection of principal and interest placed on nonaccrual status for commercial lending segment loans   90 days  
Minimum period beyond which residential mortgages and other retail loans secured by 1-4 family properties are charged down to fair value of the collateral securing the loan less costs to sell   180 days  
Minimum period beyond which revolving consumer lines and credit cards are charged off   180 days  
Minimum period beyond which other retail loans not secured by 1-4 family properties are charged off   120 days  
Foreclosed residential real estate property included in other real estate owned 114,000,000 $ 114,000,000 156,000,000
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 240,000,000 240,000,000 267,000,000
Residential mortgage loans secured by residential real estate in process of foreclosure $ 1,600,000,000 $ 1,600,000,000 1,700,000,000
Maximum restructuring period under credit card and other retail loan financial difficulty modifications   60 months  
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 240 716  
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 $ 27,000,000 $ 73,000,000  
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 1,200,000,000 $ 1,200,000,000 $ 1,300,000,000
Home Equity and Second Mortgages [Member]      
Loans and Allowance for Credit Losses [Line Items]      
Number of loans in trial period | SecurityLoan   36  
Outstanding balance of loans in trial period   $ 3,000,000  
Estimated post-modification balance of loans in trial period   3,000,000  
Commercial Lending Segment [Member] | Minimum [Member]      
Loans and Allowance for Credit Losses [Line Items]      
Impairment loan threshold for allowance in which loans are individually analyzed $ 5,000,000 $ 5,000,000  
Residential Mortgages [Member]      
Loans and Allowance for Credit Losses [Line Items]      
Number of loans in trial period | SecurityLoan   66  
Outstanding balance of loans in trial period   $ 12,000,000  
Estimated post-modification balance of loans in trial period   $ 12,000,000  
Residential Mortgages [Member] | Junior Lien [Member]      
Loans and Allowance for Credit Losses [Line Items]      
Residential mortgage loans and lines in a junior lien position secured by 1-4 family properties placed on nonaccrual status   120 days  
Residential Mortgages [Member] | First Lien [Member]      
Loans and Allowance for Credit Losses [Line Items]      
Residential mortgage loans and lines in a junior lien position placed on nonaccrual status when behind a first lien past due   180 days  
Government National Mortgage Association [Member]      
Loans and Allowance for Credit Losses [Line Items]      
Number of loans in trial period | SecurityLoan   1,127  
Outstanding balance of loans in trial period   $ 149,000,000  
Estimated post-modification balance of loans in trial period   $ 151,000,000