Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 260,093	$ 256,253
Total loans	$ 264,522	$ 260,849
Total loans, Percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 85,986	$ 83,116
Loans, excluding covered loans, percentage	32.50%	31.90%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 91,277	$ 88,402
Loans, excluding covered loans, percentage	34.50%	33.90%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 42,743	$ 42,137
Loans, excluding covered loans, percentage	16.20%	16.10%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 54,955	$ 53,496
Loans, excluding covered loans, percentage	20.80%	20.50%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 51,161	$ 51,206
Loans, excluding covered loans, percentage	19.30%	19.60%
Lease Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 5,291	$ 5,286
Loans, excluding covered loans, percentage	2.00%	2.00%
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 31,936	$ 31,773
Loans, excluding covered loans, percentage	12.10%	12.20%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 10,807	$ 10,364
Loans, excluding covered loans, percentage	4.10%	3.90%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 41,742	$ 40,425
Loans, excluding covered loans, percentage	15.80%	15.50%
Home Equity Loans, First Liens [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 13,213	$ 13,071
Loans, excluding covered loans, percentage	5.00%	5.00%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 19,957	$ 21,012
Loans, excluding covered loans, percentage	7.50%	8.10%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 5,268	$ 5,232
Loans, excluding covered loans, percentage	2.00%	2.00%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 16,332	$ 16,384
Loans, excluding covered loans, percentage	6.20%	6.30%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 3,220	$ 3,354
Loans, excluding covered loans, percentage	1.20%	1.30%
Installment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 7,162	$ 7,030
Loans, excluding covered loans, percentage	2.70%	2.70%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 16,657	$ 16,587
Loans, excluding covered loans, percentage	6.30%	6.30%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 2,522	$ 2,619
Loans, excluding covered loans, percentage	0.90%	1.00%
Total Loans, Excluding Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 260,093	$ 256,253
Loans, excluding covered loans, percentage	98.30%	98.20%
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 4,429	$ 4,596
Total loans, Percentage	1.70%	1.80%