Note 8 - Preferred Stock and Common Stock Warrants - Allocation of Net Cash Proceeds (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
	May 31, 2019	Sep. 30, 2019	Sep. 30, 2018
Proceeds from equity issuance	$ 20,945	$ 20,945	$ 79,055
Equity issuance costs
Net proceeds - Initial Fair Value Allocation	20,945
Per balance sheet upon issuance
Series B Convertible Preferred Stock [Member]
Proceeds from equity issuance
Equity issuance costs
Net proceeds - Initial Fair Value Allocation	19,009
Per balance sheet upon issuance	19,009
Series B Warrants [Member]
Proceeds from equity issuance
Equity issuance costs
Net proceeds - Initial Fair Value Allocation	1,936
Per balance sheet upon issuance	$ 1,936