Loans and asset quality - Loan Distribution and Industry Concentrations (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 64,328	$ 56,469
Unearned income on lease financings	247	274
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,700
Financial institutions
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	9,400
Other residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	317	389
Overdrafts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,600	2,700
Margin loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	20,800	15,400
Domestic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	52,112	44,553
Domestic | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,659	1,356
Domestic | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	6,081	6,056
Domestic | Financial institutions
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	4,049	4,495
Domestic | Lease financings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	297	431
Domestic | Wealth management loans and mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	17,819	16,211
Domestic | Other residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	317	389
Domestic | Overdrafts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,667	651
Domestic | Other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,308	1,823
Domestic | Margin loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	17,915	13,141
Foreign
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	12,216	11,916
Foreign | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	16	73
Foreign | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	311	0
Foreign | Financial institutions
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	5,399	6,750
Foreign | Lease financings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	476	559
Foreign | Wealth management loans and mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	168	146
Foreign | Other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,940	2,113
Foreign | Margin loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 2,906	$ 2,275