Loans and asset quality- Credit Quality Indicators - Commercial Portfolio - Credit Risk Profile by Creditworthiness Category (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 49,224	$ 46,629
Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	1,480	1,417
Commercial | Investment grade
Financing Receivable, Recorded Investment [Line Items]
Loans	1,110	1,064
Commercial | Noninvestment grade
Financing Receivable, Recorded Investment [Line Items]
Loans	370	353
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	1,874	1,740
Commercial real estate | Investment grade
Financing Receivable, Recorded Investment [Line Items]
Loans	1,493	1,289
Commercial real estate | Noninvestment grade
Financing Receivable, Recorded Investment [Line Items]
Loans	381	451
Financial institutions
Financing Receivable, Recorded Investment [Line Items]
Loans	13,784	11,288
Financial institutions | Investment grade
Financing Receivable, Recorded Investment [Line Items]
Loans	12,128	9,935
Financial institutions | Noninvestment grade
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 1,656	$ 1,353