Loans and asset quality - Allowance for Credit Losses Activity (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 319	$ 400	$ 454	$ 479	$ 216
Charge-offs	0	(1)	0	(2)	0
Recoveries	1	3	1	6	4
Net recoveries (charge-offs)	1	2	1	4	4
Provision	(47)	(83)	5	(210)	309
Ending balance	273	319	460	273	460
Allowance for loan losses	233	269	325	233	325	$ 358
Allowance for lending-related commitments	40	50	135	40	135	$ 121
Individually evaluated for impairment:
Loan balance	60	44	17	60	17
Allowance for loan losses	19	3	0	19	0
Provision for other credit losses	2	(3)	4	(4)	12
Collateral dependent loans	60			60
Fair value of collateral	52			52
Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					(69)
Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					147
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	8	11	40	16	60
Charge-offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Net recoveries (charge-offs)	0	0	0	0	0
Provision	2	(3)	(13)	(6)	10
Ending balance	10	8	27	10	27
Allowance for loan losses	2	3	14	2	14
Allowance for lending-related commitments	8	5	13	8	13
Individually evaluated for impairment:
Loan balance	0	0	0	0	0
Allowance for loan losses	0	0	0	0	0
Commercial | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					(43)
Commercial | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					17
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	289	365	372	430	76
Charge-offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Net recoveries (charge-offs)	0	0	0	0	0
Provision	(63)	(76)	14	(204)	296
Ending balance	226	289	386	226	386
Allowance for loan losses	199	248	270	199	270
Allowance for lending-related commitments	27	41	116	27	116
Individually evaluated for impairment:
Loan balance	25	26	0	25	0
Allowance for loan losses	3	3	0	3	0
Commercial real estate | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					14
Commercial real estate | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					90
Financial institutions
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	7	7	16	10	20
Charge-offs	0	0	0	0	0
Recoveries	0	2	0	2	0
Net recoveries (charge-offs)	0	2	0	2	0
Provision	2	(2)	(5)	(3)	(3)
Ending balance	9	7	11	9	11
Allowance for loan losses	5	4	7	5	7
Allowance for lending-related commitments	4	3	4	4	4
Individually evaluated for impairment:
Loan balance	0	0	0	0	0
Allowance for loan losses	0	0	0	0	0
Financial institutions | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					(6)
Financial institutions | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					14
Lease financings
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	2	2	3	2	3
Charge-offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Net recoveries (charge-offs)	0	0	0	0	0
Provision	(1)	0	0	(1)	0
Ending balance	1	2	3	1	3
Allowance for loan losses	1	2	3	1	3
Allowance for lending-related commitments	0	0	0	0	0
Individually evaluated for impairment:
Loan balance	0	0	0	0	0
Allowance for loan losses	0	0	0	0	0
Lease financings | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					0
Lease financings | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					3
Wealth management loans and mortgages
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5	6	11	8	20
Charge-offs	0	0	0	(1)	0
Recoveries	0	0	0	0	0
Net recoveries (charge-offs)	0	0	0	(1)	0
Provision	0	(1)	4	(2)	7
Ending balance	5	5	15	5	15
Allowance for loan losses	4	4	13	4	13
Allowance for lending-related commitments	1	1	2	1	2
Individually evaluated for impairment:
Loan balance	18	17	17	18	17
Allowance for loan losses	0	0	0	0	0
Collateral dependent loans		44	17		17
Fair value of collateral		50	25		25
Wealth management loans and mortgages | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					(12)
Wealth management loans and mortgages | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					8
Other residential mortgages
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	8	9	12	13	13
Charge-offs	0	(1)	0	(1)	0
Recoveries	1	1	1	4	4
Net recoveries (charge-offs)	1	0	1	3	4
Provision	(3)	(1)	5	(10)	(1)
Ending balance	6	8	18	6	18
Allowance for loan losses	6	8	18	6	18
Allowance for lending-related commitments	0	0	0	0	0
Individually evaluated for impairment:
Loan balance	1	1	0	1	0
Allowance for loan losses	0	0	0	0	0
Other residential mortgages | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					2
Other residential mortgages | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					15
Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0			0
Charge-offs	0			0
Recoveries	0			0
Net recoveries (charge-offs)	0			0
Provision	16			16
Ending balance	16	0		16
Allowance for loan losses	16			16
Allowance for lending-related commitments	0			0
Individually evaluated for impairment:
Loan balance	16			16
Allowance for loan losses	16			16
Foreign
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					24
Charge-offs					0
Recoveries					0
Net recoveries (charge-offs)					0
Provision					0
Ending balance			0		0
Foreign | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					(24)
Foreign | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance					0
Foreign | Financial institutions
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses	$ 4	$ 4	$ 8	$ 4	$ 8