Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 297,608	$ 297,707
Loans, percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 95,876	$ 97,315
Loans, percentage	32.20%	32.70%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 101,013	$ 102,871
Loans, percentage	34.00%	34.60%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 38,808	$ 39,311
Loans, percentage	13.00%	13.20%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 74,954	$ 76,155
Loans, percentage	25.20%	25.60%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 60,696	$ 57,024
Loans, percentage	20.40%	19.10%
Lease Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 5,137	$ 5,556
Loans, percentage	1.80%	1.90%
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 28,029	$ 28,472
Loans, percentage	9.40%	9.60%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 10,779	$ 10,839
Loans, percentage	3.60%	3.60%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 65,941	$ 66,525
Loans, percentage	22.20%	22.40%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 9,013	$ 9,630
Loans, percentage	3.00%	3.20%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 22,137	$ 22,346
Loans, percentage	7.40%	7.50%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 7,505	$ 8,150
Loans, percentage	2.50%	2.70%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 10,718	$ 12,472
Loans, percentage	3.60%	4.20%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 2,682	$ 2,688
Loans, percentage	0.90%	0.90%
Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 16,166	$ 13,823
Loans, percentage	5.50%	4.60%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 23,488	$ 19,722
Loans, percentage	7.90%	6.60%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 137	$ 169
Loans, percentage		0.10%