Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Mar. 31, 2019
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	$ 473,907	$ 472,146
Gross Unrealized Gains	900	375
Gross Unrealized Losses	(47)	(37)
Fair Market Value	474,760	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	53,694	60,089
Fair Market Value	53,694	60,089
Commercial Paper \| Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	49,230	56,780
Gross Unrealized Gains	29	16
Gross Unrealized Losses	(4)	(1)
Fair Market Value	49,255	56,795
U.S. Treasury mutual fund securities \| Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	28,012	58,786
Gross Unrealized Gains	39	13
Gross Unrealized Losses		(12)
Fair Market Value	28,051	58,787
U.S. Treasury mutual fund securities \| Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	6,964
Gross Unrealized Gains	6
Fair Market Value	6,970
Government-backed securities \| Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	124,117	126,336
Gross Unrealized Gains	129	60
Gross Unrealized Losses	(24)	(15)
Fair Market Value	124,222	126,381
Government-backed securities \| Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	43,898
Gross Unrealized Gains	28
Gross Unrealized Losses	(18)
Fair Market Value	43,908
Corporate Debt Securities \| Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	92,865	128,626
Gross Unrealized Gains	183	97
Gross Unrealized Losses		(9)
Fair Market Value	93,048	128,714
Corporate Debt Securities \| Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	55,127	21,529
Gross Unrealized Gains	486	189
Gross Unrealized Losses	(1)
Fair Market Value	$ 55,612	$ 21,718

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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+ References

No definition available.

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