Loans and asset quality - Allowance for Credit Losses Activity (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Accounting Standards Update [Extensible List]					us-gaap:AccountingStandardsUpdate201602Member	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201802Member
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 454		$ 241	$ 216	$ 252	$ 252
Charge-offs	0	$ 0	(1)	0	(13)
Recoveries	1	3	0	4	2
Net (charge-offs) recoveries	1	3	(1)	4	(11)
Provision	5	163	(16)	309	(17)
Ending balance	460	454	224	460	224	216	$ 252
Individually evaluated for impairment:
Loan balance	17	18	16	17	16
Allowance for loan losses	0	0	0	0	0
Collectively evaluated for impairment:
Loan balance			54,865		54,865
Allowance for loan losses			127		127
Collateral dependent loans	17	18		17
Fair value of collateral	25	26		25
Loan balance			54,865		54,865
Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	302
Ending balance	325	302	127	325	127
Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	152
Ending balance	135	152	97	135	97
Other Financial Instruments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision				12
Domestic | Overdrafts
Collectively evaluated for impairment:
Loan balance			1,247		1,247
Loan balance			1,247		1,247
Domestic | Margin loans
Collectively evaluated for impairment:
Loan balance			10,177		10,177
Loan balance			10,177		10,177
Domestic | Other
Collectively evaluated for impairment:
Loan balance			1,143		1,143
Loan balance			1,143		1,143
Foreign
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance			33	24	32	32
Charge-offs			0	0	0
Recoveries			0	0	0
Net (charge-offs) recoveries			0	0	0
Provision			(5)	0	(4)
Ending balance	0		28	0	28	24	32
Individually evaluated for impairment:
Loan balance			0		0
Allowance for loan losses			0		0
Collectively evaluated for impairment:
Loan balance			13,871		13,871
Allowance for loan losses			19		19
Loan balance			13,871		13,871
Foreign | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance			19		19
Foreign | Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance			9		9
Commercial | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	40		77	60	81	81
Charge-offs	0	0	(1)	0	(12)
Recoveries	0	0	0	0	0
Net (charge-offs) recoveries	0	0	(1)	0	(12)
Provision	(13)	14	(15)	10	(8)
Ending balance	27	40	61	27	61	60	81
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			1,335		1,335
Allowance for loan losses			10		10
Loan balance			1,335		1,335
Commercial | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	23
Ending balance	14	23	10	14	10
Commercial | Domestic | Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	17
Ending balance	13	17	51	13	51
Commercial | Foreign | Reclassification
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance			10		10
Commercial real estate | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	372		72	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	14	164	5	296	2
Ending balance	386	372	77	386	77	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			5,292		5,292
Allowance for loan losses			57		57
Loan balance			5,292		5,292
Commercial real estate | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	244
Ending balance	270	244	57	270	57
Commercial real estate | Domestic | Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	128
Ending balance	116	128	20	116	20
Financial institutions | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	16		21	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	(5)	(2)	0	(3)	(1)
Ending balance	11	16	21	11	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			4,973		4,973
Allowance for loan losses			7		7
Loan balance			4,973		4,973
Financial institutions | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11
Ending balance	7	11	7	7	7
Financial institutions | Domestic | Lending-related commitments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5
Ending balance	4	5	14	4	14
Financial institutions | Foreign
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11
Ending balance	8	11		8
Financial institutions | Foreign | Reclassification
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance			10		10
Lease financings | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	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	0	(10)	(1)	0	(2)
Ending balance	3	3	3	3	3	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			559		559
Allowance for loan losses			3		3
Loan balance			559		559
Lease financings | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3
Ending balance	3	3	3	3	3
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 | Reclassification
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance			4		4
Wealth management loans and mortgages | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	11		20	20	21	21
Charge-offs	0	0	0	0	(1)
Recoveries	0	0	0	0	0
Net (charge-offs) recoveries	0	0	0	0	(1)
Provision	4	2	0	7	0
Ending balance	15	11	20	15	20	20	21
Individually evaluated for impairment:
Loan balance	17	18	16	17	16
Allowance for loan losses	0	0	0	0	0
Collectively evaluated for impairment:
Loan balance			15,748		15,748
Allowance for loan losses			17		17
Loan balance			15,748		15,748
Wealth management loans and mortgages | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	9
Ending balance	13	9	17	13	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	12		14	13	16	16
Charge-offs	0	0	0	0	0
Recoveries	1	3	0	4	2
Net (charge-offs) recoveries	1	3	0	4	2
Provision	5	(5)	0	(1)	(4)
Ending balance	18	12	14	18	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			520		520
Allowance for loan losses			14		14
Loan balance			520		520
Other residential mortgages | Domestic | Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	12
Ending balance	18	12	14	18	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
Other Financial Instruments
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision	$ 4	(20)
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			12,567		12,567
Allowance for loan losses			0		0
Loan balance			12,567		12,567
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
Cumulative Effect, Period Of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(69)
Ending balance						(69)
Cumulative Effect, Period Of Adoption, Adjustment | Foreign
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(24)
Ending balance						(24)
Cumulative Effect, Period Of Adoption, Adjustment | Commercial | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(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
Ending balance						14
Cumulative Effect, Period Of Adoption, Adjustment | Financial institutions | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				(6)
Ending balance						(6)
Cumulative Effect, Period Of Adoption, Adjustment | Lease financings | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				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)
Ending balance						(12)
Cumulative Effect, Period Of Adoption, Adjustment | Other residential mortgages | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				2
Ending balance						2
Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		288		147
Ending balance						147
Cumulative Effect, Period of Adoption, Adjusted Balance | Foreign
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance				0
Ending balance						0
Cumulative Effect, Period of Adoption, Adjusted Balance | Commercial | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		26		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		208		90
Ending balance						90
Cumulative Effect, Period of Adoption, Adjusted Balance | Financial institutions | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		18		14
Ending balance						14
Cumulative Effect, Period of Adoption, Adjusted Balance | Lease financings | Domestic
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance		13		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		9		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		$ 14		$ 15
Ending balance						$ 15