Loans and Allowance for Credit Losses - Composition of Loan Portfolio (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 267,268	$ 256,253
Total loans	$ 271,289	$ 260,849
Total loans, Percentage	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 87,851	$ 83,116
Loans, excluding covered loans, percentage	32.40%	31.90%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 93,201	$ 88,402
Loans, excluding covered loans, percentage	34.40%	33.90%
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 43,468	$ 42,137
Loans, excluding covered loans, percentage	16.00%	16.10%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 56,229	$ 53,496
Loans, excluding covered loans, percentage	20.70%	20.50%
Other Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 53,664	$ 51,206
Loans, excluding covered loans, percentage	19.80%	19.60%
Lease Financing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 5,350	$ 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	$ 31,916	$ 31,773
Loans, excluding covered loans, percentage	11.80%	12.20%
Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 11,552	$ 10,364
Loans, excluding covered loans, percentage	4.20%	3.90%
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 42,846	$ 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	$ 13,383	$ 13,071
Loans, excluding covered loans, percentage	4.90%	5.00%
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 20,706	$ 21,012
Loans, excluding covered loans, percentage	7.60%	8.10%
Retail Leasing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 6,076	$ 5,232
Loans, excluding covered loans, percentage	2.20%	2.00%
Home Equity and Second Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 16,467	$ 16,384
Loans, excluding covered loans, percentage	6.10%	6.30%
Revolving Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 3,247	$ 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	$ 7,983	$ 7,030
Loans, excluding covered loans, percentage	2.90%	2.70%
Automobile [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 17,559	$ 16,587
Loans, excluding covered loans, percentage	6.50%	6.30%
Student [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, excluding covered loans	$ 2,332	$ 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	$ 267,268	$ 256,253
Loans, excluding covered loans, percentage	98.50%	98.20%
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 4,021	$ 4,596
Total loans, Percentage	1.50%	1.80%