Note 15 - Fair Value Measurements - Assets and Liabilities Carried at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Total fair value of assets	$ 332,838	$ 325,132
Fair Value, Inputs, Level 1 [Member]
Total fair value of assets	994	31
Fair Value, Inputs, Level 2 [Member]
Total fair value of assets	331,844	325,101
Money Market Funds [Member]
Cash equivalents	740	3,332
Money Market Funds [Member] \| Fair Value, Inputs, Level 1 [Member]
Cash equivalents	38	31
Money Market Funds [Member] \| Fair Value, Inputs, Level 2 [Member]
Cash equivalents	702	3,301
Commercial Paper [Member]
Cash equivalents	96,759	76,456
Investment in marketable securities	39,701	56,994
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	96,759	76,456
Investment in marketable securities	39,701	56,994
Municipal Bonds [Member]
Cash equivalents		999
Investment in marketable securities	6,733	27,657
Municipal Bonds [Member] \| Fair Value, Inputs, Level 1 [Member]
Cash equivalents
Investment in marketable securities
Municipal Bonds [Member] \| Fair Value, Inputs, Level 2 [Member]
Cash equivalents		999
Investment in marketable securities	6,733	27,657
Corporate Debt Securities [Member]
Cash equivalents		2,608
Investment in marketable securities	146,126	146,401
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 1 [Member]
Cash equivalents
Investment in marketable securities
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Cash equivalents		2,608
Investment in marketable securities	146,126	146,401
Variable Rate Demand Note [Member]
Investment in marketable securities	8,900	3,500
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	3,500
Asset-backed Securities [Member]
Investment in marketable securities	32,923	7,185
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	32,923	$ 7,185
Certificates of Deposit [Member]
Investment in marketable securities	956
Certificates of Deposit [Member] \| Fair Value, Inputs, Level 1 [Member]
Investment in marketable securities	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=99377789&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)
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