Revenues - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Net revenues	$ 5,840	$ 5,240	$ 16,840	$ 15,175
U.S.
Disaggregation of Revenue [Line Items]
Net revenues	2,587	2,334	7,573	6,896
International
Disaggregation of Revenue [Line Items]
Net revenues	3,253	2,906	9,267	8,279
Service revenues
Disaggregation of Revenue [Line Items]
Net revenues	2,405	2,196	7,164	6,595
Data processing revenues
Disaggregation of Revenue [Line Items]
Net revenues	2,662	2,359	7,564	6,633
International transaction revenues
Disaggregation of Revenue [Line Items]
Net revenues	1,977	1,830	5,624	5,248
Other revenues
Disaggregation of Revenue [Line Items]
Net revenues	342	229	968	688
Client incentives
Disaggregation of Revenue [Line Items]
Net revenues	$ (1,546)	$ (1,374)	$ (4,480)	$ (3,989)