Note 9 - Preferred Stock and Common Stock Warrants - Allocation of Net Cash Proceeds (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Proceeds from equity issuance	$ 70,945	$ 79,055
Equity issuance costs		(2,104)
Net proceeds - Initial Fair Value Allocation		76,951
Additional Paid-in Capital [Member] | Beneficial Conversion Feature [Member]
Net proceeds - Initial Fair Value Allocation
Per balance sheet upon issuance		2,530
Allocation to BCF		2,530
Series B Warrants [Member]
Net proceeds - Initial Fair Value Allocation	1,936	2,746
Per balance sheet upon issuance	1,936	2,746
Allocation to BCF
Series A Warrants [Member]
Net proceeds - Initial Fair Value Allocation		4,859
Per balance sheet upon issuance		4,859
Allocation to BCF
Convertible Preferred Stock [Member]
Proceeds from equity issuance	20,945
Equity issuance costs
Net proceeds - Initial Fair Value Allocation	20,945
Series B Convertible Preferred Stock [Member]
Net proceeds - Initial Fair Value Allocation	19,009	26,159
Per balance sheet upon issuance	$ 19,009	26,159
Allocation to BCF
Series A Convertible Preferred Stock [Member]
Net proceeds - Initial Fair Value Allocation		41,079
Per balance sheet upon issuance		38,549
Allocation to BCF		(2,530)
Series A Convertible Preferred Stock [Member] | Additional Paid-in Capital [Member]
Allocation to BCF		2,500
Common Stock [Member] | Additional Paid-in Capital [Member]
Net proceeds - Initial Fair Value Allocation		2,108
Per balance sheet upon issuance		2,108
Allocation to BCF