Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Mar. 31, 2019
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	$ 533,362	$ 472,146
Gross Unrealized Gains	912	375
Gross Unrealized Losses	(50)	(37)
Fair Market Value	534,224	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	36,674	60,089
Fair Market Value	36,674	60,089
Commercial Paper \| Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	39,903	56,780
Gross Unrealized Gains	9	16
Gross Unrealized Losses	(9)	(1)
Fair Market Value	39,903	56,795
U.S. Treasury mutual fund securities \| Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	37,158	58,786
Gross Unrealized Gains	23	13
Gross Unrealized Losses		(12)
Fair Market Value	37,181	58,787
U.S. Treasury mutual fund securities \| Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	10,086
Gross Unrealized Gains	2
Fair Market Value	10,088
Government-backed securities \| Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	108,117	126,336
Gross Unrealized Gains	121	60
Gross Unrealized Losses	(11)	(15)
Fair Market Value	108,227	126,381
Government-backed securities \| Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	71,963
Gross Unrealized Gains	46
Gross Unrealized Losses	(26)
Fair Market Value	71,983
Corporate Debt Securities \| Short-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	124,113	128,626
Gross Unrealized Gains	149	97
Gross Unrealized Losses	(4)	(9)
Fair Market Value	124,258	128,714
Corporate Debt Securities \| Long-term Investments
Total Cash Equivalents and Marketable Securities [Line Items]
Amortized Cost	85,348	21,529
Gross Unrealized Gains	562	189
Fair Market Value	$ 85,910	$ 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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