Note 4 - Investments in Debt Securities - Amortized Cost and Fair Value of Investments in Debt Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investment in debt securities, amortized cost	$ 11,487
Investment in debt securities, fair value	11,489
Certificates of Deposit [Member]
Investment in debt securities, amortized cost	2,400
Investment in debt securities, fair value	2,400
US Government Corporations and Agencies Securities [Member]
Investment in debt securities, amortized cost	1,612
Investment in debt securities, fair value	1,612
Corporate Debt Securities [Member]
Investment in debt securities, amortized cost	7,475
Investment in debt securities, fair value	$ 7,477

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: 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=118954083&loc=d3e26610-111562

Reference 3: 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=118939692&loc=d3e27161-111563

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant