Note 8 - Fair Value Measurements - Fair Value, Assets Measured on a Recurring Basis (Details) - Money Market Funds [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Money market mutual funds	$ 26,934	$ 26,638
Fair Value, Inputs, Level 1 [Member]
Money market mutual funds	26,934	26,638
Fair Value, Inputs, Level 2 [Member]
Money market mutual funds
Fair Value, Inputs, Level 3 [Member]
Money market mutual funds

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

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=99377789&loc=d3e19207-110258

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