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% | ||