Note 2 - Summary of Significant Accounting Policies - Antidilutive Securities (Details) - shares	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Share-based Payment Arrangement, Option [Member]
Stock options to purchase common stock (in shares)	5,593,130	3,690,902
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)	419,147	22,500

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