Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 721,615	$ 770,998
Gross Unrealized Gains	939	1,443
Gross Unrealized Losses	(205)	(74)
Fair Market Value	722,349	772,367
Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	92,996	157,297
Fair Market Value	92,996	157,297
Repurchase Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Market Value	47,000	33,000
Repurchase Agreements | Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	47,000	33,000
Fair Market Value	47,000	33,000
Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	347,254	350,379
Gross Unrealized Gains	348	623
Gross Unrealized Losses	(25)	(17)
Fair Market Value	347,577	350,985
Long Term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	281,365	263,322
Gross Unrealized Gains	591	820
Gross Unrealized Losses	(180)	(57)
Fair Market Value	281,776	264,085
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Market Value	45,996
Money Market Funds | Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	45,996	124,297
Fair Market Value	45,996	124,297
Commercial Paper | Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	44,986	45,237
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(6)	(3)
Fair Market Value	44,981	45,235
U.S. Treasury mutual fund securities | Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	110,826	72,221
Gross Unrealized Gains	12	28
Gross Unrealized Losses	(9)
Fair Market Value	110,829	72,249
U.S. Treasury mutual fund securities | Long Term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	20,402
Gross Unrealized Losses	(9)
Fair Market Value	20,393
Government-backed securities | Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	89,954	128,668
Gross Unrealized Gains	15	13
Gross Unrealized Losses	(5)	(12)
Fair Market Value	89,964	128,669
Government-backed securities | Long Term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	224,785	225,231
Gross Unrealized Gains	110	190
Gross Unrealized Losses	(141)	(37)
Fair Market Value	224,754	225,384
Corporate Debt Securities | Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	101,488	104,253
Gross Unrealized Gains	320	581
Gross Unrealized Losses	(5)	(2)
Fair Market Value	101,803	104,832
Corporate Debt Securities | Long Term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	36,178	38,091
Gross Unrealized Gains	481	630
Gross Unrealized Losses	(30)	(20)
Fair Market Value	$ 36,629	$ 38,701