Loans and Allowance for Loan Losses - Average Investment in Impaired Loans and Interest Income Recognized for Impaired Loans (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	$ 623	$ 555
Recognized Interest Income	6	3
Commercial and industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	241	198
Recognized Interest Income	3	1
Commercial mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	63	124
Recognized Interest Income
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	18	65
Recognized Interest Income		1
Commercial portfolio [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	322	387
Recognized Interest Income	3	2
Residential mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	275	154
Recognized Interest Income	3	1
Home equity and other consumer loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	21	2
Recognized Interest Income
Consumer portfolio [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	296	156
Recognized Interest Income	3	1
Loans excluding purchased credit-impaired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	618	543
Recognized Interest Income	6	3
Purchased Credit Impaired [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	5	12
Recognized Interest Income