Note 10 - Net Loss Per Share (Details Textual) - $ / shares shares in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2015
Securities Excluded from Computation of Earnings Per Share Amount, Performance Conditions (in shares)	8.3	3.0	8.3	3.0
Sale of Stock, Price Per Share (in dollars per share)					$ 17.50
Number of Trading Days of Share Price (Day)					20 days
Total Number of Trading Days Considered (Day)					30 days
IPO Warrants [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)					$ 11.50
Class of Warrant or Right, Redemption Price of Warrant or Right (in dollars per share)					$ 0.01
Class of Warrant or Right, Number of Days Notice to Redeem (Day)					30 days