Note 10 - Equity Offerings (Details Textual) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended						6 Months Ended	9 Months Ended
	Jul. 06, 2020	Mar. 10, 2020	Aug. 12, 2019	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Feb. 24, 2020	Dec. 31, 2019	Apr. 28, 2017
Shares Issued, Price Per Share (in dollars per share)															$ 1.00
Preferred Stock, Par or Stated Value Per Share (in dollars per share)				$ 0.01							$ 0.01			$ 0.01
Proceeds from Issuance of Equity Net											$ 13,525	$ 0
Proceeds from Warrant Exercises											$ 3,340	$ 539
Class of Warrant or Right, Outstanding (in shares)				46,498,909							46,498,909
Common Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)				42,857,000		7,030,000	2,545,000
Stock Issued During Period Shares Stock Options and Warrants Exercised (in shares)							1,000	25,000	12,000		4,900,000
Stock Issued During Period, Shares, Conversion of Convertible Securities (in shares)					4,884,000	3,053,000				7,900,000
Series C Warrants [Member]
Proceeds from Warrant Exercises											$ 3,300
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)		$ 0.68
Class of Warrant or Right, Outstanding (in shares)		25,367,646
Series D Warrants [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)		$ 0.68
Class of Warrant or Right, Outstanding (in shares)		25,367,646
Series B Warrant [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)				567,660							567,660
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)				$ 0.35							$ 0.35				$ 13.00
Class of Warrant or Right, Outstanding (in shares)				567,660							567,660		3,638,780
Series A Preferred Stock [Member]
Preferred Stock, Par or Stated Value Per Share (in dollars per share)		$ 0.01
The 2019 Sales Agreement [Member]
Aggregate Number of Shares of Common Stock Available for Sale in Offering (in shares)			25,000,000.0
Agency Commission as Percentage from Proceeds from Sale Of Common Stock Gross			3.00%
Public Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares)	42,857,142
Shares Issued, Price Per Share (in dollars per share)	$ 0.35
Proceeds from Issuance of Equity Net	$ 13,600	$ 13,500
Public Offering [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)						14,122,000
Public Offering [Member] | Series C Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)		3,308,823
Public Offering [Member] | Series D Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)		3,308,823
Public Offering [Member] | Class A Units [Member]
Stock Issued During Period, Shares, New Issues (in shares)		14,121,766
Shares Issued, Price Per Share (in dollars per share)		$ 0.68
Public Offering [Member] | Class B Units [Member]
Stock Issued During Period, Shares, New Issues (in shares)		7,937,057
Shares Issued, Price Per Share (in dollars per share)		$ 0.68