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