Loans and asset quality- Information about Impaired Loans (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Financing Receivable, Impaired [Line Items]
Impaired loans with an allowance, recorded investment	$ 31		$ 32
Impaired loans with an allowance, related allowance	8	[1],[2]	7	[1],[2]
Loans Individually Less Than 1 Million
Financing Receivable, Impaired [Line Items]
Impaired loans with an allowance, recorded investment	1
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] The allowance for impaired loans is included in the allowance for loan losses.

[2] Excludes an aggregate of less than $1 million of impaired loans in amounts individually less than $1 million at both March 31, 2014 and Dec. 31, 2013. The allowance for loan loss associated with these loans totaled less than $1 million at both March 31, 2014 and Dec. 31, 2013.