Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 31, 2019
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	$ 591,917	$ 472,146
Gross Unrealized Gains	1,985	375
Gross Unrealized Losses	(313)	(37)
Fair Market Value	593,589	472,484
Cash Equivalents
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	135,019	80,089
Fair Market Value	135,019	80,089
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	249,877	370,528
Gross Unrealized Gains	981	186
Gross Unrealized Losses	(83)	(37)
Fair Market Value	250,775	370,677
Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	207,021	21,529
Gross Unrealized Gains	1,004	189
Gross Unrealized Losses	(230)
Fair Market Value	207,795	21,718
Money Market Funds
Total Cash Equivalents and Marketable Securities [Line Items]
Fair Market Value	115,019	60,089
Money Market Funds | Cash Equivalents
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	115,019	60,089
Fair Market Value	115,019	60,089
Commercial Paper | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	32,757	56,780
Gross Unrealized Gains	74	16
Gross Unrealized Losses		(1)
Fair Market Value	32,831	56,795
Government-backed securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	67,594	126,336
Gross Unrealized Gains	401	60
Gross Unrealized Losses		(15)
Fair Market Value	67,995	126,381
Government-backed securities | Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	90,911
Gross Unrealized Gains	153
Fair Market Value	91,064
U.S. Treasury securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	42,236	58,786
Gross Unrealized Gains	412	13
Gross Unrealized Losses		(12)
Fair Market Value	42,648	58,787
U.S. Treasury securities | Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Fair Market Value		21,718
Corporate Debt Securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	107,290	128,626
Gross Unrealized Gains	94	97
Gross Unrealized Losses	(83)	(9)
Fair Market Value	107,301	128,714
Corporate Debt Securities | Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	116,110	21,529
Gross Unrealized Gains	851	189
Gross Unrealized Losses	(230)
Fair Market Value	$ 116,731	$ 21,718