Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Net revenues	$ 21,846	$ 22,977	$ 20,609
U.S.
Disaggregation of Revenue [Line Items]
Net revenues	10,125	10,279	9,332
International
Disaggregation of Revenue [Line Items]
Net revenues	11,721	12,698	11,277
Service revenues
Disaggregation of Revenue [Line Items]
Net revenues	9,804	9,700	8,918
Data processing revenues
Disaggregation of Revenue [Line Items]
Net revenues	10,975	10,333	9,027
International transaction revenues
Disaggregation of Revenue [Line Items]
Net revenues	6,299	7,804	7,211
Other revenues
Disaggregation of Revenue [Line Items]
Net revenues	1,432	1,313	944
Client incentives
Disaggregation of Revenue [Line Items]
Net revenues	$ (6,664)	$ (6,173)	$ (5,491)