Note 2 - Summary of Significant Accounting Policies - Antidilutive Securities (Details) - shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Share-based Payment Arrangement, Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)	5,314,375	4,209,355
Common Stock Warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)	642,622	642,622
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)	393,051	66,250