Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 250,000	$ 242,570
Total loans	$ 254,791	$ 247,851
Total loans, Percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 80,313	$ 74,996
Loans, excluding covered loans, percentage	31.50%	30.20%
Lease Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 5,226	$ 5,381
Loans, excluding covered loans, percentage	2.10%	2.20%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 85,539	$ 80,377
Loans, excluding covered loans, percentage	33.60%	32.40%
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 32,089	$ 33,360
Loans, excluding covered loans, percentage	12.60%	13.50%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 10,389	$ 9,435
Loans, excluding covered loans, percentage	4.10%	3.80%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 42,478	$ 42,795
Loans, excluding covered loans, percentage	16.70%	17.30%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 39,341	$ 38,598
Loans, excluding covered loans, percentage	15.40%	15.60%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 13,008	$ 13,021
Loans, excluding covered loans, percentage	5.10%	5.20%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 52,349	$ 51,619
Loans, excluding covered loans, percentage	20.50%	20.80%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 18,583	$ 18,515
Loans, excluding covered loans, percentage	7.30%	7.50%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 5,387	$ 5,871
Loans, excluding covered loans, percentage	2.10%	2.40%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 16,188	$ 15,916
Loans, excluding covered loans, percentage	6.30%	6.40%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 3,334	$ 3,309
Loans, excluding covered loans, percentage	1.30%	1.30%
Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 6,949	$ 6,242
Loans, excluding covered loans, percentage	2.70%	2.50%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 16,484	$ 14,822
Loans, excluding covered loans, percentage	6.50%	6.00%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 2,709	$ 3,104
Loans, excluding covered loans, percentage	1.10%	1.30%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 51,051	$ 49,264
Loans, excluding covered loans, percentage	20.00%	19.90%
Total Loans, Excluding Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 250,000	$ 242,570
Loans, excluding covered loans, percentage	98.10%	97.90%
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 4,791	$ 5,281
Total loans, Percentage	1.90%	2.10%