Revenues Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Net revenues	$ 5,506	$ 4,862
U.S.
Disaggregation of Revenue [Line Items]
Net revenues	2,508	2,265
International
Disaggregation of Revenue [Line Items]
Net revenues	2,998	2,597
Service revenues
Disaggregation of Revenue [Line Items]
Net revenues	2,342	2,146
Data processing revenues
Disaggregation of Revenue [Line Items]
Net revenues	2,470	2,147
International transaction revenues
Disaggregation of Revenue [Line Items]
Net revenues	1,851	1,666
Other revenues
Disaggregation of Revenue [Line Items]
Net revenues	299	229
Client incentives
Disaggregation of Revenue [Line Items]
Net revenues	$ (1,456)	$ (1,326)