Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 274,932	$ 277,311
Total loans	$ 277,911	$ 280,432
Total loans, Percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 92,511	$ 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	$ 98,097	$ 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	$ 40,140	$ 40,463
Loans, excluding covered loans, percentage	14.40%	14.50%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 60,477	$ 59,783
Loans, excluding covered loans, percentage	21.80%	21.30%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 55,317	$ 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	$ 5,586	$ 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	$ 28,982	$ 29,367
Loans, excluding covered loans, percentage	10.40%	10.50%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 11,158	$ 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	$ 47,583	$ 46,685
Loans, excluding covered loans, percentage	17.10%	16.60%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 12,894	$ 13,098
Loans, excluding covered loans, percentage	4.70%	4.70%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 20,901	$ 22,180
Loans, excluding covered loans, percentage	7.50%	7.90%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 8,048	$ 7,988
Loans, excluding covered loans, percentage	2.90%	2.80%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 16,030	$ 16,327
Loans, excluding covered loans, percentage	5.80%	5.80%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 3,061	$ 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	$ 9,089	$ 8,989
Loans, excluding covered loans, percentage	3.30%	3.20%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 18,762	$ 18,934
Loans, excluding covered loans, percentage	6.70%	6.80%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 327	$ 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	$ 274,932	$ 277,311
Loans, excluding covered loans, percentage	98.90%	98.90%
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 2,979	$ 3,121
Total loans, Percentage	1.10%	1.10%