Note 10 - Equity Offerings (Details Textual) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended			6 Months Ended		18 Months Ended
	Jul. 06, 2020	Mar. 10, 2020	Aug. 12, 2019	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 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	$ 0.01
Proceeds from Issuance of Equity Net							$ 13,525	$ 0
Proceeds from Warrant Exercises							$ 3,340	$ 534
Common Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)					7,030,000
Stock Issued During Period Shares Stock Options and Warrants Exercised (in shares)						25,000	4,900,000		12,000
Stock Issued During Period, Shares, Conversion of Convertible Securities (in shares)				4,884,000	3,053,000		7,900,000
Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)					0
Stock Issued During Period Shares Stock Options and Warrants Exercised (in shares)						0			0
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
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]
Proceeds from Issuance of Equity Net		$ 13,500
Public Offering [Member] | Subsequent Event [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	$ 13,700
Public Offering [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)					14,122,000
Public Offering [Member] | Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)					7,937,000
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