Loans and asset quality - Loan Distribution and Industry Concentrations (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 55,491	$ 54,953
Unearned income on lease financings	285	313
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,900
Financial institutions
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	11,000
Other residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	423	494
Overdrafts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		2,700
Margin loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	13,500	13,400
Domestic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	43,186	42,548
Domestic | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,839	1,442
Domestic | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	5,987	5,575
Domestic | Financial institutions
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,915	4,852
Domestic | Lease financings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	482	537
Domestic | Wealth management loans and mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	15,805	16,050
Domestic | Other residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	423	494
Domestic | Overdrafts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	899	524
Domestic | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,616	1,167
Domestic | Margin loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	11,220	11,907
Foreign
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	12,305	12,405
Foreign | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	102	347
Foreign | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	5	7
Foreign | Financial institutions
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	6,097	7,626
Foreign | Lease financings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	596	576
Foreign | Wealth management loans and mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	121	140
Foreign | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	3,106	2,230
Foreign | Margin loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 2,278	$ 1,479