Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 296,912	$ 297,707
Loans, percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 98,232	$ 97,315
Loans, percentage	33.10%	32.70%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 103,521	$ 102,871
Loans, percentage	34.90%	34.60%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 38,770	$ 39,311
Loans, percentage	13.10%	13.20%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 73,366	$ 76,155
Loans, percentage	24.70%	25.60%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 59,439	$ 57,024
Loans, percentage	20.00%	19.10%
Lease Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 5,289	$ 5,556
Loans, percentage	1.80%	1.90%
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 28,017	$ 28,472
Loans, percentage	9.50%	9.60%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 10,753	$ 10,839
Loans, percentage	3.60%	3.60%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 64,168	$ 66,525
Loans, percentage	21.60%	22.40%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 9,198	$ 9,630
Loans, percentage	3.10%	3.20%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 21,816	$ 22,346
Loans, percentage	7.30%	7.50%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 7,799	$ 8,150
Loans, percentage	2.60%	2.70%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 11,163	$ 12,472
Loans, percentage	3.80%	4.20%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 2,628	$ 2,688
Loans, percentage	0.90%	0.90%
Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 15,632	$ 13,823
Loans, percentage	5.30%	4.60%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 22,070	$ 19,722
Loans, percentage	7.40%	6.60%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 147	$ 169
Loans, percentage		0.10%