Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 294,638	$ 286,810
Loans, percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 99,067	$ 96,849
Loans, percentage	33.60%	33.80%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 104,654	$ 102,444
Loans, percentage	35.50%	35.70%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 39,268	$ 39,539
Loans, percentage	13.30%	13.80%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 69,378	$ 65,034
Loans, percentage	23.60%	22.70%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 57,448	$ 56,430
Loans, percentage	19.50%	19.70%
Lease Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 5,587	$ 5,595
Loans, percentage	1.90%	1.90%
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 28,664	$ 28,596
Loans, percentage	9.70%	10.00%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 10,604	$ 10,943
Loans, percentage	3.60%	3.80%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 58,465	$ 53,034
Loans, percentage	19.90%	18.50%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 10,913	$ 12,000
Loans, percentage	3.70%	4.20%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 23,890	$ 23,363
Loans, percentage	8.10%	8.10%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 8,463	$ 8,546
Loans, percentage	2.90%	3.00%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 15,453	$ 16,122
Loans, percentage	5.20%	5.60%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 2,889	$ 3,088
Loans, percentage	1.00%	1.10%
Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 10,827	$ 9,676
Loans, percentage	3.70%	3.40%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 19,583	$ 18,719
Loans, percentage	6.60%	6.50%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 233	$ 279
Loans, percentage	0.10%	0.10%