Note 4 - Fair Value Measurements - Financial Assets Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale securities	$ 164,030	$ 27,796
US Treasury Securities [Member]
Available-for-sale securities	39,771
Debt Security, Corporate, US [Member]
Available-for-sale securities	19,153	12,993
United States Commercial Paper [Member]
Available-for-sale securities	52,794	4,929
Foreign Commercial Paper [Member]
Available-for-sale securities	52,312	9,874
Fair Value, Measurements, Recurring [Member]
Total	419,721	168,623
Fair Value, Measurements, Recurring [Member] \| Foreign Commercial Paper [Member]
Available-for-sale securities	63,304	19,854
Fair Value, Measurements, Recurring [Member] \| US Treasury Securities [Member]
Available-for-sale securities	39,771
Fair Value, Measurements, Recurring [Member] \| Debt Security, Corporate, US [Member]
Available-for-sale securities	19,153	17,520
Fair Value, Measurements, Recurring [Member] \| United States Commercial Paper [Member]
Available-for-sale securities	59,788	39,863
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Total	277,476	91,386
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| US Treasury Securities [Member]
Available-for-sale securities	39,771
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Security, Corporate, US [Member]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| United States Commercial Paper [Member]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Foreign Commercial Paper [Member]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Total	142,245	77,237
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| US Treasury Securities [Member]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Security, Corporate, US [Member]
Available-for-sale securities	19,153	17,520
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| United States Commercial Paper [Member]
Available-for-sale securities	59,788	39,863
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Foreign Commercial Paper [Member]
Available-for-sale securities	63,304	19,854
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member]
Total
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| US Treasury Securities [Member]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Security, Corporate, US [Member]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| United States Commercial Paper [Member]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Foreign Commercial Paper [Member]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] \| Money Market Funds [Member]
Money market funds	237,705	91,386
Fair Value, Measurements, Recurring [Member] \| Money Market Funds [Member] \| Fair Value, Inputs, Level 1 [Member]
Money market funds	237,705	91,386
Fair Value, Measurements, Recurring [Member] \| Money Market Funds [Member] \| Fair Value, Inputs, Level 2 [Member]
Money market funds
Fair Value, Measurements, Recurring [Member] \| Money Market Funds [Member] \| Fair Value, Inputs, Level 3 [Member]
Money market funds

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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