Note 2 - Short-term Investments (Details Textual) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Short-term Investments, Total	$ 2,940,000	$ 2,695,000
Certificates of Deposit [Member] \| Minimum [Member]
Certificate of Deposit Maturity Period	120 days
Certificate of Deposit Interest Rate	2.25%
Certificates of Deposit [Member] \| Maximum [Member]
Certificate of Deposit Maturity Period	180 days
Certificate of Deposit Interest Rate	2.50%

Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(4))
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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-Section S99
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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-Subparagraph (SX 210.5-02(2))
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Name:	us-gaap_ShortTermInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant