Note 2 - Summary of Significant Accounting Policies - Antidilutive Securities (Details) - shares	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Employee Stock Option [Member]
Stock options to purchase common stock (in shares)	4,209,355	2,287,572
Common Stock Warrants [Member]
Stock options to purchase common stock (in shares)	642,622	642,622
Restricted Stock [Member]
Stock options to purchase common stock (in shares)	66,250	6,250

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Name:	us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration