Legal Matters - Additional Information (Details) $ in Millions		6 Months Ended		12 Months Ended	82 Months Ended
	Dec. 27, 2019 USD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	May 04, 2020 merchant
Loss Contingencies [Line Items]
Provision for legal matters		$ 7	$ 159
Return of takedown payment to the litigation escrow account	$ 467	467	0
Interchange Multidistrict Litigation
Loss Contingencies [Line Items]
Provision for legal matters		$ 467	$ 0
Subsequent Event | Interchange Multidistrict Litigation
Loss Contingencies [Line Items]
Settlement percentage					30.00%
Subsequent Event | U.K. Merchant Litigation
Loss Contingencies [Line Items]
Number of plaintiffs | merchant					500
Number of claims settled | merchant					100
Merchants with outstanding claims | merchant					400
Subsequent Event | U.K. Merchant Litigation | Threatened Litigation
Loss Contingencies [Line Items]
Number of plaintiffs | merchant					30
Visa, MasterCard, and Certain U.S. Financial Institutions
Loss Contingencies [Line Items]
Maximum takedown payments the defendants are entitled to				$ 700
U.S. Covered Litigation
Loss Contingencies [Line Items]
Maximum takedown payments the defendants are entitled to				467
U.S. Covered Litigation | Interchange Multidistrict Litigation
Loss Contingencies [Line Items]
Provision for legal matters				$ 600
Return of takedown payment to the litigation escrow account	$ 467