Note 7 - Financial Instruments and Investments in Securities - Investments in Held-to-maturity Securities, Recorded at Amortized Cost (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Mar. 31, 2018
Cost	$ 499,528
Estimated Fair Value	500,211
Short-term Investments [Member]
Cost	499,528	$ 279,787
Gross Unrealized Gains	683	203
Gross Unrealized Losses		(1)
Estimated Fair Value	500,211	279,989
Short-term Investments [Member] \| Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Cost	74,505	10,597
Gross Unrealized Gains	314
Gross Unrealized Losses		(1)
Estimated Fair Value	74,819	10,596
Short-term Investments [Member] \| Time Deposits [Member]
Cost	425,023	269,190
Gross Unrealized Gains	369	203
Gross Unrealized Losses
Estimated Fair Value	$ 425,392	$ 269,393

Amount of investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
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-Publisher FASB
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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

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-Name Accounting Standards Codification
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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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