Loans and Allowance for Loan Losses - Summary of Troubled Debt Restructurings (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Financing Receivable, Modifications [Line Items]
Total excluding purchased credit-impaired loans	$ 608	$ 606
Commercial and industrial [Member]
Financing Receivable, Modifications [Line Items]
Total excluding purchased credit-impaired loans	246	215
Commercial mortgage [Member]
Financing Receivable, Modifications [Line Items]
Total excluding purchased credit-impaired loans	41	64
Construction [Member]
Financing Receivable, Modifications [Line Items]
Total excluding purchased credit-impaired loans	2	35
Commercial portfolio [Member]
Financing Receivable, Modifications [Line Items]
Total excluding purchased credit-impaired loans	289	314
Residential mortgage [Member]
Financing Receivable, Modifications [Line Items]
Total excluding purchased credit-impaired loans	297	271
Home equity and other consumer loans [Member]
Financing Receivable, Modifications [Line Items]
Total excluding purchased credit-impaired loans	22	21
Consumer portfolio [Member]
Financing Receivable, Modifications [Line Items]
Total excluding purchased credit-impaired loans	$ 319	$ 292