Accounting changes and new accounting guidance - Allowance for Credit Losses (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, loans	$ 460	$ 454		$ 216	$ 224	$ 241	$ 252
Securities	12
Allowance for credit losses			$ 161
Reclassification of credit-related reserves on accounts receivable			4
Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, loans			(69)	$ (69)
Securities			7
Other			3
Allowance for credit losses			(59)
Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, loans	325	302	48		127
Lending-related commitments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, loans	$ 135	$ 152	$ 21		$ 97