Loans and asset quality - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended						6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Other Residential Mortgages Contract	Jun. 30, 2012 Wealth Management Loans Portfolio Segment Contract	Jun. 30, 2012 Wealth Management Mortgages Portfolio Segment	Dec. 31, 2011 Wealth Management Mortgages Portfolio Segment	Jun. 30, 2012 Wealth Management Mortgages Portfolio Segment New York State	Jun. 30, 2012 Wealth Management Mortgages Portfolio Segment California State	Jun. 30, 2012 Wealth Management Mortgages Portfolio Segment Massachusetts	Jun. 30, 2012 Wealth Management Mortgages Portfolio Segment Florida	Jun. 30, 2012 Wealth Management Mortgages Portfolio Segment Other	Jun. 30, 2012 Overdrafts	Dec. 31, 2011 Overdrafts	Jun. 30, 2012 Domestic	Dec. 31, 2011 Domestic	Jun. 30, 2012 Domestic Other Residential Mortgages	Dec. 31, 2011 Domestic Other Residential Mortgages	Jun. 30, 2012 Domestic Overdrafts	Mar. 31, 2012 Domestic Overdrafts	Dec. 31, 2011 Domestic Overdrafts	Jun. 30, 2011 Domestic Overdrafts	Jun. 30, 2012 Domestic Margin Loans	Mar. 31, 2012 Domestic Margin Loans	Dec. 31, 2011 Domestic Margin Loans	Jun. 30, 2011 Domestic Margin Loans	Jun. 30, 2012 Domestic Margin Loans Minimum	Dec. 31, 2011 Domestic Margin Loans Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Subsequently defaulted number of contracts			6	1
Subsequently defaulted recorded investment	$ 2
Loan to value ratio at origination					63.00%
Percentage of past due mortgages					1.00%
Geographic concentrations							23.00%	18.00%	17.00%	8.00%	34.00%
Loans	45,431	43,979			3,902	3,781						5,655	4,849	35,249	33,971	1,773	1,923	2,750	1,971	2,958	4,629	13,462	13,144	12,760	9,520
Purchased mortgages																$ 542
Purchased residential mortgages, loan to value ratio																75.00%
Percentage of purchased residential mortgages that were at least 60 days delinquent																29.00%
Number of business days in which overdrafts are generally repaid	2 days											2 days
Required daily collateral margin																										100.00%	100.00%