Note 17 - Equity Offerings (Details Textual) - USD ($)						1 Months Ended	2 Months Ended	4 Months Ended	9 Months Ended	12 Months Ended
	Jan. 29, 2021	Jul. 06, 2020	Mar. 10, 2020	Feb. 10, 2020	Sep. 04, 2019	Jan. 26, 2021	Mar. 11, 2021	Jun. 30, 2020	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Oct. 09, 2020	Feb. 24, 2020	Aug. 12, 2019	Apr. 28, 2017	Dec. 06, 2013
Proceeds from Issuance of Equity Net										$ 13,478,000	$ 0
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
Convertible Preferred Stock Converted to Other Securities										$ 299,000	$ (0)
Proceeds from Warrant Exercises										$ 3,340,000	$ 538,000
Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)										0	0
Stock Issued During Period, Shares, Conversion of Convertible Securities (in shares)								(7,900,000)		(7,937,000)
Stock Issued During Period Shares Stock Options and Warrants Exercised (in shares)										0	0
Common Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)										66,241,000	3,571,000
Stock Issued During Period, Shares, Conversion of Convertible Securities (in shares)										7,937,000
Stock Issued During Period Shares Stock Options and Warrants Exercised (in shares)									4,900,000	4,913,000	38,000
Series C Warrants [Member]
Proceeds from Warrant Exercises									$ 3,300,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)			$ 0.68
Series D Warrants [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)			0.68
Series B Warrant [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)										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				3,638,780
Series A Preferred Stock [Member]
Preferred Stock, Par or Stated Value Per Share (in dollars per share)			$ 0.01
Subsequent Event [Member]
Proceeds from Warrant Exercises							$ 30,400,000
The 2019 ATM Offering [Member]
Stock Offering Agreement, Maximum Share Value														$ 25,000,000.0
Stock Offering Agreement, Commission, Percent														3.00%
Proceeds from Issuance or Sale of Equity, Total										$ 11,558,000	$ 7,193,000
Proceeds from Issuance of Equity Net										$ 11,211,000	$ 6,981,000
Stock Issued During Period, Shares, New Issues (in shares)										6,687,846	1,374,686
Shares Issued, Price Per Share (in dollars per share)										$ 1.73	$ 5.23
The 2020 ATM Offering [Member]
Stock Offering Agreement, Maximum Share Value												$ 40,000,000.0
Stock Offering Agreement, Commission, Percent												3.00%
Proceeds from Issuance or Sale of Equity, Total										$ 9,264,000
Proceeds from Issuance of Equity Net										$ 8,986,000
Stock Issued During Period, Shares, New Issues (in shares)										16,320,793
Shares Issued, Price Per Share (in dollars per share)										$ 0.58
The 2020 ATM Offering [Member] | Subsequent Event [Member]
Proceeds from Issuance or Sale of Equity, Total						$ 28,100,000
Proceeds from Issuance of Equity Net						$ 27,257,000
Stock Issued During Period, Shares, New Issues (in shares)						19,120,037
Shares Issued, Price Per Share (in dollars per share)						$ 1.47
Public Offering [Member]
Proceeds from Issuance of Equity Net		$ 13,600,000	$ 13,500,000
Stock Issued During Period, Shares, New Issues (in shares)		42,857,142
Shares Issued, Price Per Share (in dollars per share)		$ 0.35
Public Offering [Member] | Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)										7,937,000
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
Public Offering [Member] | Subsequent Event [Member]
Stock Issued During Period, Shares, New Issues (in shares)	26,725,832
Shares Issued, Price Per Share (in dollars per share)	$ 3.00
Public Stock Offering [Member] | Underwriting Agreement [Member]
Sale of Stock, Number of Shares Issued in Transaction (in shares)					2,153,846
Over-Allotment Option [Member] | Underwriting Agreement [Member]
Proceeds from Issuance or Sale of Equity, Total										$ 18,800,000
Sale of Stock, Number of Shares Issued in Transaction (in shares)					323,077
Lincoln Park Purchase Agreement [Member]
Stock Issued During Period, Shares, New Issues (in shares)				343,171
Stock Offering Agreement, Maximum Share Value				$ 25,000,000
Purchase Agreement, Number of Additional Shares Authorized (in shares)				171,585