Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 240,183	$ 242,570
Total loans	245,301	247,851
Total loans, Percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	77,394	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	5,338	5,381
Loans, excluding covered loans, percentage	2.20%	2.20%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	82,732	80,377
Loans, excluding covered loans, percentage	33.70%	32.40%
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	32,755	33,360
Loans, excluding covered loans, percentage	13.40%	13.50%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	9,654	9,435
Loans, excluding covered loans, percentage	3.90%	3.80%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	42,409	42,795
Loans, excluding covered loans, percentage	17.30%	17.30%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	38,153	38,598
Loans, excluding covered loans, percentage	15.50%	15.60%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	12,936	13,021
Loans, excluding covered loans, percentage	5.30%	5.20%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	51,089	51,619
Loans, excluding covered loans, percentage	20.80%	20.80%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	17,504	18,515
Loans, excluding covered loans, percentage	7.20%	7.50%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	5,796	5,871
Loans, excluding covered loans, percentage	2.30%	2.40%
Home Equity And Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	15,859	15,916
Loans, excluding covered loans, percentage	6.50%	6.40%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	3,233	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	6,345	6,242
Loans, excluding covered loans, percentage	2.60%	2.50%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	15,216	14,822
Loans, excluding covered loans, percentage	6.20%	6.00%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans		3,104
Loans, excluding covered loans, percentage		1.30%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	46,449	49,264
Loans, excluding covered loans, percentage	18.90%	19.90%
Total Loans, Excluding Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	240,183	242,570
Loans, excluding covered loans, percentage	97.90%	97.90%
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 5,118	$ 5,281
Total loans, Percentage	2.10%	2.10%