Common Stock and Stockholders' Equity - Schedule of Stock Option and RSU Award Activities (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) $ / shares shares
Number of Shares
Beginning balance (in shares) | shares	2,751
Stock options granted (in shares) | shares	273
Stock options exercised (in shares) | shares	(121)
Stock options canceled/forfeited (in shares) | shares	(43)
Ending balance (in shares) | shares	2,860	2,751
Weighted- Average Exercise Price
Beginning balance (in usd per share) | $ / shares	$ 35.10
Stock options granted (in usd per share) | $ / shares	91.94
Stock options exercised (in usd per share) | $ / shares	25.22
Stock options canceled/forfeited (in usd per share) | $ / shares	67.87
Ending balance (in usd per share) | $ / shares	$ 40.45	$ 35.10
Weighted- Average Remaining Contractual Term (in years)
Weighted- Average Remaining Contractual Term (in years)	5 years 7 months 6 days	6 years 3 months 18 days
Aggregate Intrinsic Value
Aggregate intrinsic value, outstanding shares | $	$ 85,051	$ 176,002
Stock options exercised | $	$ 7,872
Number of Shares
Beginning balance (in shares) | shares	2,419
RSUs granted (in shares) | shares	521
RSUs vested (in shares) | shares	(432)
RSUs canceled/forfeited (in shares) | shares	(212)
Ending balance (in shares) | shares	2,296	2,419
Weighted- Average Grant Date Fair Value
Beginning balance (in usd per share) | $ / shares	$ 76.34
RSUs granted (in usd per share) | $ / shares	82.25
RSUs vested (in usd per share) | $ / shares	61.85
RSUs canceled/forfeited (in usd per share) | $ / shares	75.54
Ending balance (in usd per share) | $ / shares	$ 80.48	$ 76.34
Weighted- Average Remaining Contractual Term (in years)
Weighted Average Remaining Contractual Terms (in years)	2 years 8 months 12 days	3 years
Aggregate Intrinsic Value
Aggregate intrinsic value, outstanding | $	$ 141,140	$ 238,734