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Summary of Weighted Average Grant-Date Fair Values And Weighted Average Assumptions Used to Calculate Fair Value of Stock Options Granted (Detail) - Stock Options - $ / shares
3 Months Ended 9 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average grant-date fair value $ 117.50 $ 70.14 $ 142.08 $ 51.20
Valuation assumptions:        
Risk-free interest rate 2.81% 2.13% 2.93% 1.86%
Expected option life (years) 4 years 14 days 4 years 29 days 4 years 14 days 4 years 25 days
Expected volatility 45.20% 42.20% 42.80% 43.60%