Accounting changes and new accounting guidance - Allowance for Credit Losses (Details) - USD ($) $ in Millions	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, loans	$ 288	$ 147	$ 216	$ 224	$ 248	$ 252
Securities	15
ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, loans		(69)
Allowance for credit losses		161
Cumulative Effect, Period Of Adoption, Adjustment | ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, loans		(69)
Securities		7
Other		3
Allowance for credit losses		(59)
Reclassification of credit-related reserves on accounts receivable		4
Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, loans	140		122		146
Loans | ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, loans		48
Lending-related commitments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, loans	$ 148		$ 94		$ 102
Lending-related commitments | ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, loans		$ 21