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Summary of Weighted Average Grant-Date Fair Values And Weighted Average Assumptions Used to Calculate Fair Value of Options Granted (Detail) - Stock Options - $ / shares
3 Months Ended 6 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average grant-date fair value $ 148.51 $ 57.38 $ 142.81 $ 50.26
Valuation assumptions:        
Risk-free interest rate 2.87% 1.88% 2.93% 1.85%
Expected option life (years) 3 years 11 months 26 days 4 years 21 days 4 years 14 days 4 years 25 days
Expected volatility 43.90% 43.40% 42.70% 43.60%