Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	$ 528,788	$ 591,917
Gross Unrealized Gains	3,433	1,985
Gross Unrealized Losses	(5)	(313)
Fair Market Value	532,216	593,589
Cash Equivalents
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	150,012	135,019
Fair Market Value	150,012	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	211,425	249,877
Gross Unrealized Gains	1,111	981
Gross Unrealized Losses		(83)
Fair Market Value	212,536	250,775
Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	167,351	207,021
Gross Unrealized Gains	2,322	1,004
Gross Unrealized Losses	(5)	(230)
Fair Market Value	169,668	207,795
Money Market Funds
Total Cash Equivalents and Marketable Securities [Line Items]
Fair Market Value	130,012	115,019
Money Market Funds | Cash Equivalents
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	130,012	115,019
Fair Market Value	130,012	115,019
Commercial Paper | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	19,898	32,757
Gross Unrealized Gains	86	74
Fair Market Value	19,984	32,831
U.S. Treasury mutual fund securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	42,180	42,236
Gross Unrealized Gains	227	412
Fair Market Value	42,407	42,648
Government-backed securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	39,921	67,594
Gross Unrealized Gains	210	401
Fair Market Value	40,131	67,995
Government-backed securities | Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	75,483	90,911
Gross Unrealized Gains	66	153
Gross Unrealized Losses	(5)
Fair Market Value	75,544	91,064
Corporate Debt Securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	109,426	107,290
Gross Unrealized Gains	588	94
Gross Unrealized Losses		(83)
Fair Market Value	110,014	107,301
Corporate Debt Securities | Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	91,868	116,110
Gross Unrealized Gains	2,256	851
Gross Unrealized Losses		(230)
Fair Market Value	$ 94,124	$ 116,731