Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 280,061	$ 277,311
Total loans	$ 281,461	$ 280,432
Total loans, Percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 93,692	$ 91,958
Loans, excluding covered loans, percentage	33.30%	32.80%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 99,273	$ 97,561
Loans, excluding covered loans, percentage	35.30%	34.80%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 39,966	$ 40,463
Loans, excluding covered loans, percentage	14.20%	14.50%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 62,904	$ 59,783
Loans, excluding covered loans, percentage	22.30%	21.30%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 56,049	$ 57,324
Loans, excluding covered loans, percentage	19.90%	20.40%
Lease Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 5,581	$ 5,603
Loans, excluding covered loans, percentage	2.00%	2.00%
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 28,633	$ 29,367
Loans, excluding covered loans, percentage	10.20%	10.50%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 11,333	$ 11,096
Loans, excluding covered loans, percentage	4.00%	4.00%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 50,614	$ 46,685
Loans, excluding covered loans, percentage	18.00%	16.60%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 12,290	$ 13,098
Loans, excluding covered loans, percentage	4.30%	4.70%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 21,869	$ 22,180
Loans, excluding covered loans, percentage	7.80%	7.90%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 8,447	$ 7,988
Loans, excluding covered loans, percentage	3.00%	2.80%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 15,966	$ 16,327
Loans, excluding covered loans, percentage	5.70%	5.80%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 3,129	$ 3,183
Loans, excluding covered loans, percentage	1.10%	1.10%
Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 9,666	$ 8,989
Loans, excluding covered loans, percentage	3.40%	3.20%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 18,547	$ 18,934
Loans, excluding covered loans, percentage	6.60%	6.80%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 294	$ 1,903
Loans, excluding covered loans, percentage	0.10%	0.70%
Total Loans, Excluding Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 280,061	$ 277,311
Loans, excluding covered loans, percentage	99.50%	98.90%
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 1,400	$ 3,121
Total loans, Percentage	0.50%	1.10%