Commitments and contingent liabilities - Narrative (Details)				9 Months Ended
	Sep. 09, 2020 USD ($)	Dec. 19, 2019 claim	Dec. 17, 2015 lawsuit	Sep. 30, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	Dec. 31, 2009 claim
Commitments and Contingencies Disclosure [Line Items]
Lending commitment maturing in less than one year				$ 31,000,000,000.0
Lending commitment maturing in one to five years				16,300,000,000
Lending commitment maturing over five years				$ 336,000,000
Collateralization percentage generally required for a securities lending transaction with indemnification against broker default				102.00%
Securities lending indemnifications, secured amount of collateral				$ 435,000,000,000	$ 428,000,000,000
Securities lending indemnifications, joint venture				58,000,000,000	57,000,000,000
Securities lending indemnifications, collateral joint venture				$ 61,000,000,000	61,000,000,000
Matters Related to R. Allen Stanford
Commitments and Contingencies Disclosure [Line Items]
Number of lawsuits or class actions pending | claim						2
Number of claims dismissed | claim		6
Number of lawsuits filed | claim				13
Brazilian Postalis Litigation
Commitments and Contingencies Disclosure [Line Items]
Number of lawsuits filed | lawsuit			3
Loss contingency, damages awarded, value	$ 41,700,000
Loss contingency, damages sought, value	$ 1,800,000
Maximum
Commitments and Contingencies Disclosure [Line Items]
Loss contingency, aggregate range of reasonable loss (up to)				$ 750,000,000
Standby letters of credit
Commitments and Contingencies Disclosure [Line Items]
Standby letters of credit (SBLC) collateralized with cash and securities				200,000,000	184,000,000
SBLC expiring within one year				1,600,000,000
SBLC expiring within one to five years				568,000,000
SBLC expiring after five years				1,000,000
Off-balance sheet credit risks				2,175,000,000	2,298,000,000
Commercial letters of credit
Commitments and Contingencies Disclosure [Line Items]
Off-balance sheet credit risks				87,000,000	74,000,000
Unsettled Repurchase Agreements
Commitments and Contingencies Disclosure [Line Items]
Guarantor obligations, maximum exposure, undiscounted				150,000,000
Unsettled Reverse Repurchase Agreements
Commitments and Contingencies Disclosure [Line Items]
Guarantor obligations, maximum exposure, undiscounted				0
Operating segments
Commitments and Contingencies Disclosure [Line Items]
Allowance for credit losses on lending related commitments				$ 135,000,000	$ 94,000,000
Federal Court | Mortgage-Securitization Trusts Proceedings
Commitments and Contingencies Disclosure [Line Items]
Number of lawsuits or class actions pending | claim				3
State Court | Mortgage-Securitization Trusts Proceedings
Commitments and Contingencies Disclosure [Line Items]
Number of lawsuits or class actions pending | claim				1