Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 275,457	$ 269,371
Total loans	$ 278,719	$ 273,207
Total loans, Percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 91,449	$ 87,928
Loans, excluding covered loans, percentage	32.80%	32.20%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 96,928	$ 93,386
Loans, excluding covered loans, percentage	34.80%	34.20%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 41,430	$ 43,098
Loans, excluding covered loans, percentage	14.80%	15.80%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 59,317	$ 57,274
Loans, excluding covered loans, percentage	21.30%	21.00%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 56,859	$ 53,864
Loans, excluding covered loans, percentage	20.40%	19.70%
Lease Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 5,479	$ 5,458
Loans, excluding covered loans, percentage	2.00%	2.00%
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 29,902	$ 31,592
Loans, excluding covered loans, percentage	10.70%	11.60%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 11,528	$ 11,506
Loans, excluding covered loans, percentage	4.10%	4.20%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 46,107	$ 43,632
Loans, excluding covered loans, percentage	16.60%	16.00%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 13,210	$ 13,642
Loans, excluding covered loans, percentage	4.70%	5.00%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 20,923	$ 21,749
Loans, excluding covered loans, percentage	7.50%	7.90%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 7,923	$ 6,316
Loans, excluding covered loans, percentage	2.80%	2.30%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 16,308	$ 16,369
Loans, excluding covered loans, percentage	5.90%	6.00%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 3,225	$ 3,282
Loans, excluding covered loans, percentage	1.20%	1.20%
Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 8,900	$ 8,087
Loans, excluding covered loans, percentage	3.20%	3.00%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 18,530	$ 17,571
Loans, excluding covered loans, percentage	6.60%	6.40%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 1,973	$ 2,239
Loans, excluding covered loans, percentage	0.70%	0.80%
Total Loans, Excluding Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 275,457	$ 269,371
Loans, excluding covered loans, percentage	98.80%	98.60%
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 3,262	$ 3,836
Total loans, Percentage	1.20%	1.40%