Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 310,335	$ 296,102
Loans, percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 114,621	$ 98,168
Loans, percentage	37.00%	33.20%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 120,261	$ 103,863
Loans, percentage	38.80%	35.10%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 41,076	$ 39,746
Loans, percentage	13.20%	13.40%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 71,329	$ 70,586
Loans, percentage	23.00%	23.80%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 56,412	$ 57,118
Loans, percentage	18.20%	19.30%
Lease Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 5,640	$ 5,695
Loans, percentage	1.80%	1.90%
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 30,098	$ 29,404
Loans, percentage	9.70%	9.90%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 10,978	$ 10,342
Loans, percentage	3.50%	3.50%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 61,169	$ 59,865
Loans, percentage	19.70%	20.20%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 10,160	$ 10,721
Loans, percentage	3.30%	3.60%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 21,257	$ 24,789
Loans, percentage	6.80%	8.40%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 8,412	$ 8,490
Loans, percentage	2.70%	2.90%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 13,932	$ 15,036
Loans, percentage	4.50%	5.10%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 2,625	$ 2,899
Loans, percentage	0.80%	1.00%
Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 12,556	$ 11,038
Loans, percentage	4.10%	3.70%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 18,694	$ 19,435
Loans, percentage	6.00%	6.50%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 193	$ 220
Loans, percentage	0.10%	0.10%