Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 273,846	$ 269,371
Total loans	$ 277,283	$ 273,207
Total loans, Percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 91,212	$ 87,928
Loans, excluding covered loans, percentage	32.90%	32.20%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 96,836	$ 93,386
Loans, excluding covered loans, percentage	34.90%	34.20%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 41,908	$ 43,098
Loans, excluding covered loans, percentage	15.10%	15.80%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 58,796	$ 57,274
Loans, excluding covered loans, percentage	21.20%	21.00%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 55,445	$ 53,864
Loans, excluding covered loans, percentage	20.00%	19.70%
Lease Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 5,624	$ 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	$ 30,198	$ 31,592
Loans, excluding covered loans, percentage	10.90%	11.60%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 11,710	$ 11,506
Loans, excluding covered loans, percentage	4.20%	4.20%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 45,412	$ 43,632
Loans, excluding covered loans, percentage	16.40%	16.00%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 13,384	$ 13,642
Loans, excluding covered loans, percentage	4.80%	5.00%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 20,861	$ 21,749
Loans, excluding covered loans, percentage	7.60%	7.90%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 7,569	$ 6,316
Loans, excluding covered loans, percentage	2.70%	2.30%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 16,310	$ 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	$ 3,209	$ 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	$ 8,602	$ 8,087
Loans, excluding covered loans, percentage	3.10%	3.00%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 17,695	$ 17,571
Loans, excluding covered loans, percentage	6.40%	6.40%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 2,060	$ 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	$ 273,846	$ 269,371
Loans, excluding covered loans, percentage	98.80%	98.60%
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 3,437	$ 3,836
Total loans, Percentage	1.20%	1.40%