Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 306,985	$ 296,102
Loans, percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 105,109	$ 98,168
Loans, percentage	34.20%	33.20%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 110,764	$ 103,863
Loans, percentage	36.10%	35.10%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 40,380	$ 39,746
Loans, percentage	13.20%	13.40%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 76,789	$ 70,586
Loans, percentage	25.00%	23.80%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 57,154	$ 57,118
Loans, percentage	18.60%	19.30%
Lease Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 5,655	$ 5,695
Loans, percentage	1.90%	1.90%
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 29,264	$ 29,404
Loans, percentage	9.60%	9.90%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 11,116	$ 10,342
Loans, percentage	3.60%	3.50%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 66,952	$ 59,865
Loans, percentage	21.80%	20.20%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 9,837	$ 10,721
Loans, percentage	3.20%	3.60%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 21,898	$ 24,789
Loans, percentage	7.10%	8.40%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 8,405	$ 8,490
Loans, percentage	2.70%	2.90%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 13,208	$ 15,036
Loans, percentage	4.30%	5.10%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 2,660	$ 2,899
Loans, percentage	0.90%	1.00%
Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 13,513	$ 11,038
Loans, percentage	4.40%	3.70%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 19,188	$ 19,435
Loans, percentage	6.20%	6.50%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 180	$ 220
Loans, percentage	0.10%	0.10%