Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	$ 655,150	$ 591,917
Gross Unrealized Gains	2,632	1,985
Gross Unrealized Losses	(21)	(313)
Fair Market Value	657,761	593,589
Cash Equivalents
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	124,305	135,019
Fair Market Value	124,305	135,019
Repurchase Agreement
Total Cash Equivalents and Marketable Securities [Line Items]
Fair Market Value	20,000	20,000
Repurchase Agreement | Cash Equivalents
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	20,000	20,000
Fair Market Value	20,000	20,000
Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	329,594	249,877
Gross Unrealized Gains	1,264	981
Gross Unrealized Losses	(15)	(83)
Fair Market Value	330,843	250,775
Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	201,251	207,021
Gross Unrealized Gains	1,368	1,004
Gross Unrealized Losses	(6)	(230)
Fair Market Value	202,613	207,795
Money Market Funds
Total Cash Equivalents and Marketable Securities [Line Items]
Fair Market Value	104,305	115,019
Money Market Funds | Cash Equivalents
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	104,305	115,019
Fair Market Value	104,305	115,019
Commercial Paper | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	39,991	32,757
Gross Unrealized Gains	31	74
Gross Unrealized Losses	(5)
Fair Market Value	40,017	32,831
U.S. Treasury mutual fund securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	75,505	42,236
Gross Unrealized Gains	85	412
Gross Unrealized Losses	(1)
Fair Market Value	75,589	42,648
U.S. Treasury mutual fund securities | Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	10,246
Gross Unrealized Gains	2
Fair Market Value	10,248
Government-backed securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	85,360	67,594
Gross Unrealized Gains	105	401
Gross Unrealized Losses	(9)
Fair Market Value	85,456	67,995
Government-backed securities | Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	141,378	90,911
Gross Unrealized Gains	164	153
Gross Unrealized Losses	(6)
Fair Market Value	141,536	91,064
Corporate Debt Securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	128,738	107,290
Gross Unrealized Gains	1,043	94
Gross Unrealized Losses		(83)
Fair Market Value	129,781	107,301
Corporate Debt Securities | Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	49,627	116,110
Gross Unrealized Gains	1,202	851
Gross Unrealized Losses		(230)
Fair Market Value	$ 50,829	$ 116,731