Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Disaggregation of Revenue [Line Items]
Net revenue	$ 9,594	$ 8,775	$ 19,104	$ 17,409
U.S.
Disaggregation of Revenue [Line Items]
Net revenue	3,811	3,643	7,549	7,288
International
Disaggregation of Revenue [Line Items]
Net revenue	5,783	5,132	11,555	10,121
Service revenue
Disaggregation of Revenue [Line Items]
Net revenue	4,399	4,033	8,607	7,948
Data processing revenue
Disaggregation of Revenue [Line Items]
Net revenue	4,701	4,259	9,446	8,615
International transaction revenue
Disaggregation of Revenue [Line Items]
Net revenue	3,291	2,984	6,733	6,003
Other revenue
Disaggregation of Revenue [Line Items]
Net revenue	937	756	1,849	1,448
Client incentives
Disaggregation of Revenue [Line Items]
Net revenue	$ (3,734)	$ (3,257)	$ (7,531)	$ (6,605)