Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2022	Mar. 31, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 871,380	$ 915,660
Gross Unrealized Gains	20	37
Gross Unrealized Losses	(11,750)	(7,906)
Fair Market Value	859,650	907,791
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Market Value	86,806	32,955
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Market Value		28,958
U.S. Treasury mutual fund securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	17,939
Gross Unrealized Gains	0
Gross Unrealized Losses	(3)
Fair Market Value	17,936
Short-term U.S. Treasury mutual fund securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	335,024	287,010
Gross Unrealized Gains	6	0
Gross Unrealized Losses	(3,218)	(1,384)
Fair Market Value	331,812	285,626
Short-term government-backed securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	146,747	131,954
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(2,550)	(554)
Fair Market Value	144,197	131,401
Short-term corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	40,681	61,108
Gross Unrealized Gains	0	36
Gross Unrealized Losses	(519)	(113)
Fair Market Value	40,162	61,031
Short-term commercial paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	122,353	148,128
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(487)	(397)
Fair Market Value	121,866	147,731
Short-term marketable securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	644,805	628,200
Gross Unrealized Gains	6	37
Gross Unrealized Losses	(6,774)	(2,448)
Fair Market Value	638,037	625,789
Long-term U.S. Treasury mutual fund securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	28,595	89,168
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,229)	(1,796)
Fair Market Value	27,366	87,372
Long-term government-backed securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	93,235	126,150
Gross Unrealized Gains	14	0
Gross Unrealized Losses	(3,744)	(3,378)
Fair Market Value	89,505	122,772
Long-term corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		10,226
Gross Unrealized Gains		0
Gross Unrealized Losses		(281)
Fair Market Value		9,945
Long-term marketable securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	121,830	225,544
Gross Unrealized Gains	14	0
Gross Unrealized Losses	(4,973)	(5,455)
Fair Market Value	116,871	220,089
Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	104,745	61,916
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3)	(3)
Fair Market Value	104,742	61,913
Cash Equivalents | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	86,806	32,955
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	$ 86,806	32,955
Cash Equivalents | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		28,961
Gross Unrealized Gains		0
Gross Unrealized Losses		(3)
Fair Market Value		$ 28,958