Common Stock and Stockholders' Equity - Narrative (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Dec. 31, 2014	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Class of Stock [Line Items]
Stock-based compensation expense		$ 23,358	$ 13,833	$ 44,546	$ 24,860
Unrecognized stock-based compensation cost related to outstanding unvested stock awards		26,600		$ 26,600
Stock-based compensation cost expected to be recognized over weighted-average period				2 years 8 months 12 days
Expected to vest unrecognized stock-based compensation cost related to outstanding unvested stock options		166,800		$ 166,800
Unvested Stock Options
Class of Stock [Line Items]
Stock based compensation cost is expected to be recognized over a weighted-average period				2 years 9 months 18 days
ESPP shares
Class of Stock [Line Items]
Shares reserved for issuance under plan (in shares)	1,000,000
Additional shares available for issuance under the plan (in shares)	500,000
Percentage of common stock shares increased under the plan	1.00%
Stock-based compensation expense		$ 1,500	$ 1,100	$ 2,700	$ 1,500
Shares available for issuance (in shares)		2,656,874		2,656,874
2008 Equity Incentive Plan
Class of Stock [Line Items]
Shares available for issuance (in shares)		0		0
2014 Equity Incentive Plan
Class of Stock [Line Items]
Shares reserved for issuance under plan (in shares)	5,000,000
Common stock purchased under Employee Stock Purchase Plan (in shares)				117,115	82,851
Shares available for issuance (in shares)		13,162,855		13,162,855
Shares available for grant under plan (in shares)	5,184,878
Increase in shares available under plan, percentage of outstanding common stock shares	5.00%