Note 9 - Basic and Diluted Net Loss per Share - Antidilutive Securities Excluded from Computation of Earnings Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)	8,743,586	53,819,880
Share-based Payment Arrangement, Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)	4,731,249	4,361,872
Warrant [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)	1,323,995	46,498,909
Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)	2,688,342	2,959,099