Settlement Guarantee Management - Additional Information (Detail) $ in Millions	12 Months Ended
	Sep. 30, 2017 USD ($) month	Sep. 30, 2016 USD ($)
Settlement Guarantee Management [Abstract]
Number of Rolling Months of Average Chargebacks Volume Included in Settlement Risk Exposure | month	4
Estimated maximum settlement exposure	$ 67,700	$ 67,800
Covered settlement exposure	2,800	2,900
Estimated probability-weighted value of the guarantee	$ 3	$ 2