Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Mar. 31, 2020
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	$ 720,243	$ 591,917
Gross Unrealized Gains	2,094	1,985
Gross Unrealized Losses	(9)	(313)
Fair Market Value	722,328	593,589
Cash Equivalents
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	115,579	135,019
Fair Market Value	115,579	135,019
Repurchase Agreement
Total Cash Equivalents and Marketable Securities [Line Items]
Fair Market Value	33,000	20,000
Repurchase Agreement | Cash Equivalents
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	33,000	20,000
Fair Market Value	33,000	20,000
Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	304,643	249,877
Gross Unrealized Gains	798	981
Gross Unrealized Losses	(7)	(83)
Fair Market Value	305,434	250,775
Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	300,021	207,021
Gross Unrealized Gains	1,296	1,004
Gross Unrealized Losses	(2)	(230)
Fair Market Value	301,315	207,795
Money Market Funds
Total Cash Equivalents and Marketable Securities [Line Items]
Fair Market Value	82,579	115,019
Money Market Funds | Cash Equivalents
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	82,579	115,019
Fair Market Value	82,579	115,019
Commercial Paper | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	48,026	32,757
Gross Unrealized Gains	1	74
Gross Unrealized Losses	(4)
Fair Market Value	48,023	32,831
U.S. Treasury mutual fund securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	77,718	42,236
Gross Unrealized Gains	16	412
Fair Market Value	77,734	42,648
Government-backed securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	90,002	67,594
Gross Unrealized Gains	48	401
Gross Unrealized Losses	(3)
Fair Market Value	90,047	67,995
Government-backed securities | Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	251,297	90,911
Gross Unrealized Gains	356	153
Gross Unrealized Losses	(1)
Fair Market Value	251,652	91,064
Corporate Debt Securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	88,897	107,290
Gross Unrealized Gains	733	94
Gross Unrealized Losses		(83)
Fair Market Value	89,630	107,301
Corporate Debt Securities | Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	48,724	116,110
Gross Unrealized Gains	940	851
Gross Unrealized Losses	(1)	(230)
Fair Market Value	$ 49,663	$ 116,731