Loans and asset quality - Past Due Loans (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Total past due loans	$ 37	$ 87
30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	36	37
60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	1	50
Equal to Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	0	0
Domestic | Wealth management loans and mortgages
Financing Receivable, Past Due [Line Items]
Total past due loans	21	27
Domestic | Wealth management loans and mortgages | 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	20	22
Domestic | Wealth management loans and mortgages | 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	1	5
Domestic | Wealth management loans and mortgages | Equal to Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	0	0
Domestic | Other residential mortgages
Financing Receivable, Past Due [Line Items]
Total past due loans	7	11
Domestic | Other residential mortgages | 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	7	8
Domestic | Other residential mortgages | 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	0	3
Domestic | Other residential mortgages | Equal to Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	0	0
Domestic | Financial institutions
Financing Receivable, Past Due [Line Items]
Total past due loans	0	31
Domestic | Financial institutions | 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	0	1
Domestic | Financial institutions | 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	0	30
Domestic | Financial institutions | Equal to Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	0	0
Domestic | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total past due loans	9	18
Domestic | Commercial real estate | 30 to 59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	9	6
Domestic | Commercial real estate | 60 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	0	12
Domestic | Commercial real estate | Equal to Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total past due loans	$ 0	$ 0