Loans and asset quality- Information about Impaired Loans (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Impaired loans with an allowance, recorded investment	$ 94		$ 110
Impaired loans with an allowance, related allowance	20	[1],[2]	24	[1],[2]
Loans Individually Less than 1 Million
Financing Receivable, Impaired [Line Items]
Impaired loans with an allowance, recorded investment	1		2
Maximum | Loans Individually Less than 1 Million
Financing Receivable, Impaired [Line Items]
Impaired loans with an allowance, recorded investment	1		1
Impaired loans with an allowance, related allowance	$ 1		$ 1

[1] Excludes an aggregate of $1 million and $2 million of impaired loans in amounts individually less than $1 million at March 31, 2013 and Dec. 31, 2012, respectively. The allowance for loan loss associated with these loans totaled less than $1 million at both March 31, 2013 and Dec. 31, 2012.

[2] The allowance for impaired loans is included in the allowance for loan losses.