Note 14 - Fair Value Measurements - Assets and Liabilities Carried at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Total fair value of assets	$ 341,896	$ 332,838
Fair Value, Inputs, Level 1 [Member]
Total fair value of assets	928	994
Fair Value, Inputs, Level 2 [Member]
Total fair value of assets	340,968	331,844
Money Market Funds [Member]
Cash equivalents	7,442	740
Money Market Funds [Member] \| Fair Value, Inputs, Level 1 [Member]
Cash equivalents	110	38
Money Market Funds [Member] \| Fair Value, Inputs, Level 2 [Member]
Cash equivalents	7,332	702
Commercial Paper [Member]
Cash equivalents	123,448	96,759
Investment in marketable securities	17,432	39,701
Commercial Paper [Member] \| Fair Value, Inputs, Level 1 [Member]
Cash equivalents
Investment in marketable securities
Commercial Paper [Member] \| Fair Value, Inputs, Level 2 [Member]
Cash equivalents	123,448	96,759
Investment in marketable securities	17,432	39,701
Municipal Bonds [Member]
Investment in marketable securities	6,908	6,733
Municipal Bonds [Member] \| Fair Value, Inputs, Level 1 [Member]
Investment in marketable securities
Municipal Bonds [Member] \| Fair Value, Inputs, Level 2 [Member]
Investment in marketable securities	6,908	6,733
Corporate Debt Securities [Member]
Investment in marketable securities	146,321	146,126
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 1 [Member]
Investment in marketable securities
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Investment in marketable securities	146,321	146,126
Variable Rate Demand Note [Member]
Investment in marketable securities	8,900	8,900
Variable Rate Demand Note [Member] \| Fair Value, Inputs, Level 1 [Member]
Investment in marketable securities
Variable Rate Demand Note [Member] \| Fair Value, Inputs, Level 2 [Member]
Investment in marketable securities	8,900	8,900
Asset-backed Securities [Member]
Investment in marketable securities	30,627	32,923
Asset-backed Securities [Member] \| Fair Value, Inputs, Level 1 [Member]
Investment in marketable securities
Asset-backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Investment in marketable securities	30,627	32,923
Certificates of Deposit [Member]
Investment in marketable securities	818	956
Certificates of Deposit [Member] \| Fair Value, Inputs, Level 1 [Member]
Investment in marketable securities	818	956
Certificates of Deposit [Member] \| Fair Value, Inputs, Level 2 [Member]
Investment in marketable securities

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=117815213&loc=d3e19207-110258

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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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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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