Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Net revenues	$ 5,854	$ 5,494	$ 11,908	$ 11,000
U.S.
Disaggregation of Revenue [Line Items]
Net revenues	2,650	2,479	5,367	4,987
International
Disaggregation of Revenue [Line Items]
Net revenues	3,204	3,015	6,541	6,013
Service revenues
Disaggregation of Revenue [Line Items]
Net revenues	2,623	2,417	5,178	4,759
Data processing revenues
Disaggregation of Revenue [Line Items]
Net revenues	2,711	2,432	5,575	4,902
International transaction revenues
Disaggregation of Revenue [Line Items]
Net revenues	1,833	1,796	3,851	3,647
Other revenues
Disaggregation of Revenue [Line Items]
Net revenues	392	327	757	626
Client incentives
Disaggregation of Revenue [Line Items]
Net revenues	$ (1,705)	$ (1,478)	$ (3,453)	$ (2,934)