Fair Value Measurements - Information about financial assets measured at fair value on recurring basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	$ 681,628	$ 678,815
Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	113,047	160,600
Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	562,939	512,574
Restricted cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	5,642	5,641
Money market funds | Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	113,047	107,663
Certificates of deposit | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	26,713	14,986
Commercial paper | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	20,788	13,334
Corporate notes and bonds | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	51,881	60,319
U.S. treasury securities | Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value		27,938
U.S. treasury securities | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	463,557	422,333
U.S. government agencies | Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value		24,999
U.S. government agencies | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value		1,602
Money market funds | Restricted cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	5,642	5,641
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	582,246	563,575
Level 1 | Money market funds | Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	113,047	107,663
Level 1 | Certificates of deposit | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	0
Level 1 | Commercial paper | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	0
Level 1 | Corporate notes and bonds | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	0
Level 1 | U.S. treasury securities | Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value		27,938
Level 1 | U.S. treasury securities | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	463,557	422,333
Level 1 | U.S. government agencies | Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value		0
Level 1 | U.S. government agencies | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value		0
Level 1 | Money market funds | Restricted cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	5,642	5,641
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	99,382	115,240
Level 2 | Money market funds | Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	0
Level 2 | Certificates of deposit | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	26,713	14,986
Level 2 | Commercial paper | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	20,788	13,334
Level 2 | Corporate notes and bonds | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	51,881	60,319
Level 2 | U.S. treasury securities | Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value		0
Level 2 | U.S. treasury securities | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	0
Level 2 | U.S. government agencies | Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value		24,999
Level 2 | U.S. government agencies | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value		1,602
Level 2 | Money market funds | Restricted cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	0
Level 3 | Money market funds | Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	0
Level 3 | Certificates of deposit | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	0
Level 3 | Commercial paper | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	0
Level 3 | Corporate notes and bonds | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	0
Level 3 | U.S. treasury securities | Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value		0
Level 3 | U.S. treasury securities | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	0
Level 3 | U.S. government agencies | Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value		0
Level 3 | U.S. government agencies | Short-term investments:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value		0
Level 3 | Money market funds | Restricted cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	$ 0	$ 0