Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Mar. 31, 2021
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 787,126	$ 770,998
Gross Unrealized Gains	647	1,443
Gross Unrealized Losses	(107)	(74)
Fair Market Value	787,666	772,367
Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	168,010	157,297
Fair Market Value	168,010	157,297
Repurchase Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Market Value	75,998	33,000
Repurchase Agreements | Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	75,998	33,000
Fair Market Value	75,998	33,000
Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	408,500	350,379
Gross Unrealized Gains	193	623
Gross Unrealized Losses	(16)	(17)
Fair Market Value	408,677	350,985
Long Term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	210,616	263,322
Gross Unrealized Gains	454	820
Gross Unrealized Losses	(91)	(57)
Fair Market Value	210,979	264,085
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Market Value	92,012	124,297
Money Market Funds | Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	92,012	124,297
Fair Market Value	92,012	124,297
Commercial Paper | Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	195,339	45,237
Gross Unrealized Gains	13	1
Gross Unrealized Losses	(5)	(3)
Fair Market Value	195,347	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	73,893	72,221
Gross Unrealized Gains	5	28
Gross Unrealized Losses	(2)
Fair Market Value	73,896	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	19,349
Gross Unrealized Losses	(6)
Fair Market Value	19,343
Government-backed securities | Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	73,672	128,668
Gross Unrealized Gains	49	13
Gross Unrealized Losses	(3)	(12)
Fair Market Value	73,718	128,669
Government-backed securities | Long Term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	152,698	225,231
Gross Unrealized Gains	91	190
Gross Unrealized Losses	(63)	(37)
Fair Market Value	152,726	225,384
Corporate Debt Securities | Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	65,596	104,253
Gross Unrealized Gains	126	581
Gross Unrealized Losses	(6)	(2)
Fair Market Value	65,716	104,832
Corporate Debt Securities | Long Term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	38,569	38,091
Gross Unrealized Gains	363	630
Gross Unrealized Losses	(22)	(20)
Fair Market Value	$ 38,910	$ 38,701