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Estimated Fair Value of Options on Dates of Each Grant Using Binomial Option Pricing Model (Detail)
12 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2018
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield 0.61% 0.62% 0.65%
Expected volatility 15.30% 14.53% 19.94%
Risk-free interest rate, lower range 5.81% 7.23% 6.73%
Expected term (in years) 2 years 9 months 25 days 2 years 9 months 10 days 4 years 7 months 28 days
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield 0.85% 0.65% 0.66%
Expected volatility 20.13% 18.68% 21.65%
Risk-free interest rate, upper range 6.70% 8.31% 7.20%
Expected term (in years) 5 years 5 months 1 day 5 years 1 month 28 days 6 years 21 days