Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Mar. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 770,998	$ 591,917
Gross Unrealized Gains	1,443	1,985
Gross Unrealized Losses	(74)	(313)
Fair Market Value	772,367	593,589
Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	157,297	135,019
Fair Market Value	157,297	135,019
Repurchase Agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	33,000	20,000
Repurchase Agreement | Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	33,000	20,000
Fair Market Value	33,000	20,000
Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	350,379	249,877
Gross Unrealized Gains	623	981
Gross Unrealized Losses	(17)	(83)
Fair Market Value	350,985	250,775
Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	263,322	207,021
Gross Unrealized Gains	820	1,004
Gross Unrealized Losses	(57)	(230)
Fair Market Value	264,085	207,795
Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value	124,297	115,019
Money Market Funds | Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	124,297	115,019
Fair Market Value	124,297	115,019
Commercial Paper | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	45,237	32,757
Gross Unrealized Gains	1	74
Gross Unrealized Losses	(3)
Fair Market Value	45,235	32,831
U.S. Treasury mutual fund securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	72,221	42,236
Gross Unrealized Gains	28	412
Fair Market Value	72,249	42,648
Government-backed securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	128,668	67,594
Gross Unrealized Gains	13	401
Gross Unrealized Losses	(12)
Fair Market Value	128,669	67,995
Government-backed securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	225,231	90,911
Gross Unrealized Gains	190	153
Gross Unrealized Losses	(37)
Fair Market Value	225,384	91,064
Corporate Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	104,253	107,290
Gross Unrealized Gains	581	94
Gross Unrealized Losses	(2)	(83)
Fair Market Value	104,832	107,301
Corporate Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	38,091	116,110
Gross Unrealized Gains	630	851
Gross Unrealized Losses	(20)	(230)
Fair Market Value	$ 38,701	$ 116,731