Fair Value Measurements - Information about Financial Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2019	Mar. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	$ 668,428	$ 662,389
Cash and cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	133,702	143,212
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	529,068	510,372
Restricted cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	5,658	8,805
Money market funds | Cash and cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	126,909	133,239
Commercial paper | Cash and cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	6,793	9,973
Commercial paper | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	15,883	37,071
Certificates of deposit | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	33,867	24,726
Corporate notes and bonds | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	40,592	27,259
U.S. treasury securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	429,111	402,091
U.S. government agencies | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	9,615	19,225
Money market funds | Restricted cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	5,658	8,805
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	561,678	544,135
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	126,909	133,239
Level 1 | Commercial paper | Cash and cash equivalents
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 | 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 | 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 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	429,111	402,091
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	0
Level 1 | Money market funds | Restricted cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	5,658	8,805
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	106,750	118,254
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 | Commercial paper | Cash and cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	6,793	9,973
Level 2 | Commercial paper | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	15,883	37,071
Level 2 | Certificates of deposit | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	33,867	24,726
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	40,592	27,259
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 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	9,615	19,225
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 | Commercial paper | Cash and cash equivalents
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 | 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 | 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 | 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 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value	0	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