Loans and asset quality - Allowance for Credit Losses Activity (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Accounting Standards Update [Extensible List]				us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201802Member
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 288	$ 216	$ 248	$ 216	$ 252	$ 252
Charge-offs	0	0	(1)	0	(12)
Recoveries	3	0	2	3	2
Net (charge-offs) recoveries	3	0	1	3	(10)
Provision	163	141	(8)	304	(1)
Ending balance	454	288	241	454	241	216	$ 252
Individually evaluated for impairment:
Loan balance	18	18	112	18	112
Allowance for loan losses	0	0	10	0	10
Collectively evaluated for impairment:
Loan balance			52,284		52,284
Allowance for loan losses			136		136
Collateral dependent loans	18	18		18
Fair value of collateral	26	26		26
Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	140
Ending balance	302	140	146	302	146
Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	148
Ending balance	152	148	95	152	95
Domestic | Overdrafts
Collectively evaluated for impairment:
Loans			1,575		1,575
Domestic | Margin loans
Collectively evaluated for impairment:
Loans			10,152		10,152
Domestic | Other
Collectively evaluated for impairment:
Loans			1,122		1,122
Foreign
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0	24	29	24	32	32
Charge-offs		0	0	0	0
Recoveries		0	0	0	0
Net (charge-offs) recoveries		0	0	0	0
Provision		0	4	0	1
Ending balance	0	0	33	0	33	24	32
Individually evaluated for impairment:
Loan balance		0	0		0
Allowance for loan losses		0	0		0
Collectively evaluated for impairment:
Loan balance			11,539		11,539
Allowance for loan losses			23		23
Foreign | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Ending balance		0	23		23
Foreign | Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Ending balance		0	10		10
Commercial | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	26	60	82	60	81	81
Charge-offs	0	0	0	0	(11)
Recoveries	0	0	0	0	0
Net (charge-offs) recoveries	0	0	0	0	(11)
Provision	14	9	(5)	23	7
Ending balance	40	26	77	40	77	60	81
Individually evaluated for impairment:
Loan balance	0	0	96	0	96
Allowance for loan losses	0	0	10	0	10
Collectively evaluated for impairment:
Loan balance			1,356		1,356
Allowance for loan losses			13		13
Commercial | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13
Ending balance	23	13	23	23	23
Commercial | Domestic | Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13
Ending balance	17	13	54	17	54
Commercial | Foreign
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	10
Ending balance		10
Commercial real estate | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	208	76	74	76	75	75
Charge-offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Net (charge-offs) recoveries	0	0	0	0	0
Provision	164	118	(2)	282	(3)
Ending balance	372	208	72	372	72	76	75
Individually evaluated for impairment:
Loan balance	0	0	0	0	0
Allowance for loan losses	0	0	0	0	0
Collectively evaluated for impairment:
Loan balance			5,192		5,192
Allowance for loan losses			57		57
Commercial real estate | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	83
Ending balance	244	83	57	244	57
Commercial real estate | Domestic | Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	125
Ending balance	128	125	15	128	15
Financial institutions | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	18	20	23	20	22	22
Charge-offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Net (charge-offs) recoveries	0	0	0	0	0
Provision	(2)	4	(2)	2	(1)
Ending balance	16	18	21	16	21	20	22
Individually evaluated for impairment:
Loan balance	0	0	0	0	0
Allowance for loan losses	0	0	0	0	0
Collectively evaluated for impairment:
Loan balance			4,574		4,574
Allowance for loan losses			8		8
Financial institutions | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	10
Ending balance	11	10	8	11	8
Financial institutions | Domestic | Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	8
Ending balance	5	8	13	5	13
Financial institutions | Foreign
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	10
Ending balance	11	10		11
Lease financings | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13	3	4	3	5	5
Charge-offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Net (charge-offs) recoveries	0	0	0	0	0
Provision	(10)	10	0	0	(1)
Ending balance	3	13	4	3	4	3	5
Individually evaluated for impairment:
Loan balance	0	0	0	0	0
Allowance for loan losses	0	0	0	0	0
Collectively evaluated for impairment:
Loan balance			662		662
Allowance for loan losses			4		4
Lease financings | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13
Ending balance	3	13	4	3	4
Lease financings | Domestic | Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Ending balance	0	0	0	0	0
Lease financings | Foreign
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4
Ending balance		4
Wealth management loans and mortgages | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	9	20	21	20	21	21
Charge-offs	0	0	(1)	0	(1)
Recoveries	0	0	0	0	0
Net (charge-offs) recoveries	0	0	(1)	0	(1)
Provision	2	1	0	3	0
Ending balance	11	9	20	11	20	20	21
Individually evaluated for impairment:
Loan balance	18	18	16	18	16
Allowance for loan losses	0	0	0	0	0
Collectively evaluated for impairment:
Loan balance			15,563		15,563
Allowance for loan losses			17		17
Wealth management loans and mortgages | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	7
Ending balance	9	7	17	9	17
Wealth management loans and mortgages | Domestic | Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2
Ending balance	2	2	3	2	3
Other residential mortgages | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	14	13	15	13	16	16
Charge-offs	0	0	0	0	0
Recoveries	3	0	2	3	2
Net (charge-offs) recoveries	3	0	2	3	2
Provision	(5)	(1)	(3)	(6)	(4)
Ending balance	12	14	14	12	14	13	16
Individually evaluated for impairment:
Loan balance	0	0	0	0	0
Allowance for loan losses	0	0	0	0	0
Collectively evaluated for impairment:
Loan balance			549		549
Allowance for loan losses			14		14
Other residential mortgages | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	14
Ending balance	12	14	14	12	14
Other residential mortgages | Domestic | Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Ending balance	0	0	0	0	0
All other | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance			0		0	0
Charge-offs			0		0
Recoveries			0		0
Net (charge-offs) recoveries			0		0
Provision			0		0
Ending balance			0		0		$ 0
Individually evaluated for impairment:
Loan balance			0		0
Allowance for loan losses			0		0
Collectively evaluated for impairment:
Loan balance			12,849		12,849
Allowance for loan losses			0		0
All other | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance			0		0
All other | Domestic | Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance			$ 0		$ 0
Financial Institutions and Other | Foreign
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 12
Ending balance		12
Cumulative Effect, Period Of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		(69)		(69)
Ending balance						(69)
Cumulative Effect, Period Of Adoption, Adjustment | Foreign
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		(24)		(24)
Ending balance						(24)
Cumulative Effect, Period Of Adoption, Adjustment | Commercial | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		(43)		(43)
Ending balance						(43)
Cumulative Effect, Period Of Adoption, Adjustment | Commercial real estate | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		14		14
Ending balance						14
Cumulative Effect, Period Of Adoption, Adjustment | Financial institutions | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		(6)		(6)
Ending balance						(6)
Cumulative Effect, Period Of Adoption, Adjustment | Lease financings | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		0		0
Ending balance						0
Cumulative Effect, Period Of Adoption, Adjustment | Wealth management loans and mortgages | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		(12)		(12)
Ending balance						(12)
Cumulative Effect, Period Of Adoption, Adjustment | Other residential mortgages | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		2		2
Ending balance						2
Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		147		147
Ending balance						147
Cumulative Effect, Period of Adoption, Adjusted Balance | Foreign
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		0		0
Ending balance						0
Cumulative Effect, Period of Adoption, Adjusted Balance | Commercial | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		17		17
Ending balance						17
Cumulative Effect, Period of Adoption, Adjusted Balance | Commercial real estate | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		90		90
Ending balance						90
Cumulative Effect, Period of Adoption, Adjusted Balance | Financial institutions | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		14		14
Ending balance						14
Cumulative Effect, Period of Adoption, Adjusted Balance | Lease financings | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		3		3
Ending balance						3
Cumulative Effect, Period of Adoption, Adjusted Balance | Wealth management loans and mortgages | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		8		8
Ending balance						8
Cumulative Effect, Period of Adoption, Adjusted Balance | Other residential mortgages | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		$ 15		$ 15
Ending balance						$ 15