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, 2017	Mar. 31, 2016	Mar. 31, 2015
Valuation assumptions:
Weighted average grant-date fair value	$ 42.40	$ 29.57	$ 9.29
Risk-free interest rate	1.41%	1.55%	1.60%
Expected option life (years)	4 years 1 month 21 days	4 years 1 month 24 days	4 years 2 months 9 days
Expected volatility	48.90%	49.70%	49.30%