Note 8 - Equity Offerings (Details Textual) - USD ($)					3 Months Ended				4 Months Ended	6 Months Ended		12 Months Ended
	Jan. 29, 2021	Jan. 12, 2021	Jul. 06, 2020	Mar. 10, 2020	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	May 19, 2021	Oct. 09, 2020	Aug. 12, 2019	Apr. 28, 2017
Proceeds from Issuance of Equity Net										$ 0	$ 13,525,000
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
Preferred Stock Dividends, Income Statement Impact					$ (0)		$ (0)			$ (0)	412,000	$ 400,000
Convertible Preferred Stock Converted to Other Securities							$ 299,000			(0)	299,000	$ 300,000
Proceeds from Warrant Exercises										30,600,000
Stock Issued During Period, Value, New Issues					$ 992,000	$ 129,322,000		$ 11,212,000
Proceeds from Issuance of Common Stock, Net										$ 130,314,000	$ 11,212,000
Stock Issued During Period, Shares, Warrants Exercised (in shares)										45,314,101
Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)					0	0		0
Stock Issued During Period, Shares, Conversion of Convertible Securities (in shares)							(4,884,000)	(3,053,000)	(7,900,000)
Stock Issued During Period Shares Stock Options and Warrants Exercised (in shares)					0	0	0
Stock Issued During Period, Value, New Issues					$ 0	$ 0		$ 0
Common Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)					332,000	70,666,000		7,030,000
Stock Issued During Period, Shares, Conversion of Convertible Securities (in shares)							4,884,000	3,053,000
Stock Issued During Period Shares Stock Options and Warrants Exercised (in shares)					508,000	45,114,000	4,913,000					4,900,000
Stock Issued During Period, Value, New Issues					$ 0	$ 71,000		$ 7,000
Series C Warrants [Member]
Proceeds from Warrant Exercises												$ 3,300,000
Series B Warrant [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share)					$ 0.35					$ 0.35		$ 0.35					$ 13.00
Class of Warrant or Right, Outstanding (in shares)					567,660					567,660
Series A Preferred Stock [Member]
Preferred Stock, Par or Stated Value Per Share (in dollars per share)				$ 0.01
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												$ 11,600,000	$ 7,200,000
Proceeds from Issuance of Equity Net												$ 11,200,000	$ 7,000,000.0
The 2020 ATM Offering [Member]
Stock Offering Agreement, Maximum Share Value															$ 40,000,000.0
Stock Offering Agreement, Commission, Percent															3.00%
The 2021 ATM Offering [Member]
Stock Offering Agreement, Maximum Share Value														$ 100,000,000
Stock Offering Agreement, Commission, Percent														3.00%
Proceeds from Issuance or Sale of Equity, Total										$ 1,152,000
Proceeds from Issuance of Equity Net										$ 1,118,000
Stock Issued During Period, Shares, New Issues (in shares)										331,811
Shares Issued, Price Per Share (in dollars per share)					$ 3.47					$ 3.47
Public Offering [Member]
Proceeds from Issuance of Equity Net			$ 13,600,000	$ 13,500,000
Stock Issued During Period, Shares, New Issues (in shares)	26,545,832		42,857,142
Shares Issued, Price Per Share (in dollars per share)	$ 3.00		$ 0.35
Stock Issued During Period, Value, New Issues								$ 13,525,000
Proceeds from Issuance of Common Stock	$ 79,600,000					73,400,000
Proceeds from Issuance of Common Stock, Net	$ 73,400,000
Public Offering [Member] | Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)								7,937,000
Stock Issued During Period, Value, New Issues								$ 79,000
Public Offering [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)								14,122,000
Stock Issued During Period, Value, New Issues								$ 14,000
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
Registered Direct Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares)		25,000,000
Shares Issued, Price Per Share (in dollars per share)		$ 1.25
Stock Issued During Period, Value, New Issues		$ 31,250,000
Proceeds from Issuance of Common Stock		$ 28,600,000				$ 28,600,000