Summary of Weighted Average Grant-Date Fair Values And Weighted Average Assumptions Used to Calculate Fair Value of Options Granted (Detail) - Stock Options - $ / shares	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Valuation assumptions:
Weighted average grant-date fair value	$ 52.34	$ 42.40	$ 29.57
Risk-free interest rate	1.87%	1.41%	1.55%
Expected option life (years)	4 years 25 days	4 years 1 month 20 days	4 years 1 month 24 days
Expected volatility	43.50%	48.90%	49.70%