Note 9 - Basic and Diluted Net Loss per Share - Antidilutive Securities Excluded from Computation of Earnings Per Share (Details) - shares	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)	7,064,579	53,776,773
Share-based Payment Arrangement, Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)	4,279,335	4,278,967
Warrant [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)	1,016,383	46,461,630
Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)	1,768,861	3,036,176