Investable Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 336,646	$ 380,183
Gross Unrealized Gains		7
Gross Unrealized Losses	(621)	(819)
Fair Market Value	336,025	379,371
Repurchase Agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		16,750
Fair Market Value		16,750
Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	29,910	5,845
Fair Market Value	29,910	5,845
Commercial Paper \| Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	29,549	36,936
Gross Unrealized Gains		2
Gross Unrealized Losses	(16)	(63)
Fair Market Value	29,533	36,875
U.S. Treasury mutual fund securities \| Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	23,024	18,132
Gross Unrealized Losses	(44)	(29)
Fair Market Value	22,980	18,103
U.S. Treasury mutual fund securities \| Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	6,888	10,953
Gross Unrealized Losses	(1)	(16)
Fair Market Value	6,887	10,937
Government-backed securities \| Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	210,836	212,255
Gross Unrealized Gains		3
Gross Unrealized Losses	(463)	(538)
Fair Market Value	210,373	211,720
Government-backed securities \| Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		24,798
Gross Unrealized Gains		1
Gross Unrealized Losses		(12)
Fair Market Value		24,787
Corporate Debt Securities \| Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	36,439	52,737
Gross Unrealized Losses	(97)	(161)
Fair Market Value	$ 36,342	52,576
Corporate Debt Securities \| Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		1,777
Gross Unrealized Gains		1
Fair Market Value		$ 1,778

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=82897076&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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