Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	$ 471,151	$ 472,146
Gross Unrealized Gains	939	375
Gross Unrealized Losses	(1)	(37)
Fair Market Value	472,089	472,484
Repurchase Agreement
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	20,000	20,000
Fair Market Value	20,000	20,000
Money Market Funds
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	26,433	60,089
Fair Market Value	26,433	60,089
Commercial Paper | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	58,319	56,780
Gross Unrealized Gains	73	16
Gross Unrealized Losses		(1)
Fair Market Value	58,392	56,795
U.S. Treasury mutual fund securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	47,166	58,786
Gross Unrealized Gains	57	13
Gross Unrealized Losses		(12)
Fair Market Value	47,223	58,787
Government-backed securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	152,317	126,336
Gross Unrealized Gains	191	60
Gross Unrealized Losses		(15)
Fair Market Value	152,508	126,381
Government-backed securities | Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	3,982
Gross Unrealized Gains	5
Fair Market Value	3,987
Corporate Debt Securities | Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	127,268	128,626
Gross Unrealized Gains	234	97
Gross Unrealized Losses	(1)	(9)
Fair Market Value	127,501	128,714
Corporate Debt Securities | Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	35,666	21,529
Gross Unrealized Gains	379	189
Fair Market Value	$ 36,045	$ 21,718