Revenues - Schedule of Net Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Net revenues	$ 5,729	$ 5,854	$ 11,416	$ 11,908
U.S.
Disaggregation of Revenue [Line Items]
Net revenues	2,683	2,650	5,350	5,367
International
Disaggregation of Revenue [Line Items]
Net revenues	3,046	3,204	6,066	6,541
Service revenues
Disaggregation of Revenue [Line Items]
Net revenues	2,845	2,623	5,522	5,178
Data processing revenues
Disaggregation of Revenue [Line Items]
Net revenues	2,996	2,711	6,029	5,575
International transaction revenues
Disaggregation of Revenue [Line Items]
Net revenues	1,488	1,833	2,939	3,851
Other revenues
Disaggregation of Revenue [Line Items]
Net revenues	392	392	776	757
Client incentives
Disaggregation of Revenue [Line Items]
Net revenues	$ (1,992)	$ (1,705)	$ (3,850)	$ (3,453)