Note 9 - Net Loss Per Share (Details Textual) - $ / shares shares in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2015
Securities Excluded from Computation of Earnings Per Share Amount, Performance Conditions	3.2	16.3	2.1	16.3
Sale of Stock, Price Per Share					$ 17.50
Number of Trading Days of Share Price					20 days
Total Number of Trading Days Considered					30 days
IPO Warrants [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights					$ 11.50
Class of Warrant or Right, Redemption Price of Warrant or Right					$ 0.01
Class of Warrant or Right, Number of Days Notice to Redeem					30 days