Share-based Compensation - Assumptions Used to Estimate the Fair Value of Each Stock Option on the Date of Grant Using a Black-Scholes Option Pricing Model (Detail) - Employee Stock Option - $ / shares	6 Months Ended	12 Months Ended
	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		3 years 11 months 23 days	4 years	4 years 2 months 23 days
Risk-free rate of return		2.90%	2.00%	1.60%
Expected volatility		20.20%	18.30%	20.20%
Expected dividend yield		0.70%	0.70%	0.80%
Fair value per option granted (in dollars per share)		$ 25.89	$ 18.24	$ 13.90
Expiration period of options issued under EIP		10 years
Visa
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term weighted percentage	97.00%			87.00%
Peer Companies
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term weighted percentage	3.00%			13.00%