Loans and asset quality - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended											3 Months Ended
	Mar. 31, 2012 Contract		Dec. 31, 2011		Mar. 31, 2012 Wealth Management Mortgages Portfolio Segment	Dec. 31, 2011 Wealth Management Mortgages Portfolio Segment	Mar. 31, 2012 Wealth Management Mortgages Portfolio Segment New York State	Mar. 31, 2012 Wealth Management Mortgages Portfolio Segment Massachusetts	Mar. 31, 2012 Wealth Management Mortgages Portfolio Segment California State	Mar. 31, 2012 Wealth Management Mortgages Portfolio Segment Florida	Mar. 31, 2012 Wealth Management Mortgages Portfolio Segment Other	Mar. 31, 2012 Overdrafts	Dec. 31, 2011 Overdrafts	Mar. 31, 2012 Subsequently Defaulted Other Residential Mortgages Contract	Mar. 31, 2012 Domestic	Dec. 31, 2011 Domestic	Mar. 31, 2012 Domestic Other Residential Mortgages Contract	Dec. 31, 2011 Domestic Other Residential Mortgages	Mar. 31, 2012 Domestic Overdrafts	Dec. 31, 2011 Domestic Overdrafts	Mar. 31, 2011 Domestic Overdrafts	Mar. 31, 2012 Domestic Margin Loans	Dec. 31, 2011 Domestic Margin Loans	Mar. 31, 2011 Domestic Margin Loans	Mar. 31, 2012 Domestic Margin Loans Minimum	Dec. 31, 2011 Domestic Margin Loans Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of contracts	34													7			30
Recorded investment	$ 159	[1]	$ 128	[1]										$ 2
Loan to value ratio at origination					62.00%
Percentage of past due mortgages					1.00%
Geographic concentrations							23.00%	17.00%	18.00%	8.00%	34.00%
Loans	43,028		43,979		3,830	3,781						3,715	4,849		33,747	33,971	1,854	1,923	1,971	2,958	3,381	13,144	12,760	9,369
Purchased mortgages																	$ 570
Purchased residential mortgages, loan to value ratio																	76.00%
Percentage of purchased residential mortgages that were at least 60 days delinquent																	28.00%
Number of business days in which overdrafts are generally repaid	2 days											2 days
Required daily collateral margin																									100.00%	100.00%

[1] Excludes an aggregate of $2 million of impaired loans in amounts individually less than $1 million at both March 31, 2012 and Dec. 31, 2011. The allowance for loan loss associated with these loans totaled less than $1 million at both March 31, 2012 and Dec. 31, 2011.