Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 287,699	$ 286,810
Loans, percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 97,552	$ 96,849
Loans, percentage	33.90%	33.80%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 103,069	$ 102,444
Loans, percentage	35.80%	35.70%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 39,421	$ 39,539
Loans, percentage	13.70%	13.90%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 66,243	$ 65,034
Loans, percentage	23.00%	22.70%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 56,698	$ 56,430
Loans, percentage	19.70%	19.70%
Lease Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 5,517	$ 5,595
Loans, percentage	1.90%	2.00%
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 28,416	$ 28,596
Loans, percentage	9.90%	10.00%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 11,005	$ 10,943
Loans, percentage	3.80%	3.80%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 54,552	$ 53,034
Loans, percentage	18.90%	18.50%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 11,691	$ 12,000
Loans, percentage	4.10%	4.20%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 22,268	$ 23,363
Loans, percentage	7.80%	8.10%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 8,612	$ 8,546
Loans, percentage	3.00%	3.00%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 15,883	$ 16,122
Loans, percentage	5.50%	5.60%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 2,934	$ 3,088
Loans, percentage	1.00%	1.10%
Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 10,030	$ 9,676
Loans, percentage	3.50%	3.40%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 18,976	$ 18,719
Loans, percentage	6.60%	6.50%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 263	$ 279
Loans, percentage	0.10%	0.10%