Changes in the U.S. Litigation Escrow Account (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Escrow Account [Roll Forward]
Litigation Escrow Account Balance, Beginning of Period	$ 1,027	$ 1,072
Litigation Escrow Account Balance, End of Period	1,031	1,027
Interest Income | Opt-out Merchants
Escrow Account [Roll Forward]
Payments to opt-out merchants and interest earned on escrow funds	$ 4	$ (45)