Commitments and contingent liabilities - Additional Information (Detail) (USD $)											3 Months Ended
	Mar. 31, 2012 LegalMatter	Dec. 31, 2011	Jan. 18, 2008	Mar. 31, 2012 Standby letters of credit		Dec. 31, 2011 Standby letters of credit		Mar. 31, 2012 Support agreements	Dec. 31, 2011 Support agreements	Mar. 31, 2012 United Kingdom	Mar. 31, 2012 Federal Reserve Bank of Boston
Commitments and Contingencies Disclosure [Line Items]
Lending commitment maturing in less than one year	$ 9,700,000,000
Lending commitment maturing in one to five years	18,800,000,000
Lending commitment maturing over five years	200,000,000
Potential exposure of off-balance sheet credit risks				6,399,000,000	[1]	6,707,000,000	[1]	70,000,000	63,000,000
SBLCs collateralized with cash and securities	560,000,000	485,000,000
SBLCs expiring within one year	4,400,000,000
SBLCs expiring within one to five years	2,000,000,000
Allowance for lending related commitments	108,000,000	103,000,000
Commercial Letters of Credit	294,000,000	437,000,000
Maximum maturities of prearranged contract for a securities lending transaction (in days)	90 days	90 days
Cash collateralization percentage generally required for a securities lending transaction with indemnification against broker default	102.00%	102.00%
Securities lending indemnifications, secured amount of collateral	272,000,000,000	276,000,000,000
Custody cash on deposit at other institutions as a result of GIS acquisition	690,000,000
Legal proceedings loss contingency, range of possible loss, maximum	900,000,000
Bank filed a proof of claim on Jan. 18, 2008 in the Chapter 11 bankruptcy of Sentinel Management Group, Inc. (Sentinel), seeking to recover approximate amount loaned to Sentinel			312,000,000
Number of cases regarding customer losses in the securities lending program seeking to proceed as a class action	3
Number of court actions that most of the Madoff cases have been consolidated into	2
The total amount of disputed tax assessments for the life of a transaction, covering the period from 2001 - 2006 related to foreign tax credits										900,000,000
Assessed civil money penalty											$ 6,000,000

[1] Net of participations totaling $1.1 billion at March 31, 2012 and $1.2 billion at Dec. 31, 2011.