Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Mar. 31, 2021
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 860,062	$ 770,998
Gross Unrealized Gains	240	1,443
Gross Unrealized Losses	(1,458)	(74)
Fair Market Value	858,844	772,367
Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	71,245	157,297
Gross Unrealized Losses	(2)
Fair Market Value	71,243	157,297
Repurchase Agreements | Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	10,000	33,000
Fair Market Value	10,000	33,000
Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	524,996	350,379
Gross Unrealized Gains	239	623
Gross Unrealized Losses	(269)	(17)
Fair Market Value	524,966	350,985
Long Term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	263,821	263,322
Gross Unrealized Gains	1	820
Gross Unrealized Losses	(1,187)	(57)
Fair Market Value	262,635	264,085
Money Market Funds | Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	61,245	124,297
Gross Unrealized Losses	(2)
Fair Market Value	61,243	124,297
Commercial Paper | Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	191,373	45,237
Gross Unrealized Gains	4	1
Gross Unrealized Losses	(59)	(3)
Fair Market Value	191,318	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	143,488	72,221
Gross Unrealized Gains	1	28
Gross Unrealized Losses	(101)
Fair Market Value	143,388	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	128,705
Gross Unrealized Losses	(357)
Fair Market Value	128,348
Government-backed securities | Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	126,586	128,668
Gross Unrealized Gains	1	13
Gross Unrealized Losses	(88)	(12)
Fair Market Value	126,499	128,669
Government-backed securities | Long Term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	114,681	225,231
Gross Unrealized Gains	1	190
Gross Unrealized Losses	(725)	(37)
Fair Market Value	113,957	225,384
Corporate Debt Securities | Short-term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	63,549	104,253
Gross Unrealized Gains	233	581
Gross Unrealized Losses	(21)	(2)
Fair Market Value	63,761	104,832
Corporate Debt Securities | Long Term Investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	20,435	38,091
Gross Unrealized Gains		630
Gross Unrealized Losses	(105)	(20)
Fair Market Value	$ 20,330	$ 38,701