Note 8 - Equity Offerings (Details Textual) - USD ($) $ / shares in Units, $ in Thousands					3 Months Ended		4 Months Ended	12 Months Ended
	Jan. 29, 2021	Jan. 12, 2021	Jul. 06, 2020	Mar. 10, 2020	Mar. 31, 2021	Mar. 31, 2020	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Oct. 09, 2020	Sep. 30, 2020	Aug. 12, 2019	Apr. 28, 2017
Proceeds from Issuance of Equity Net					$ 0	$ 13,525
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
Convertible Preferred Stock Converted to Other Securities								$ 300
Proceeds from Warrant Exercises					$ 30,497	0
Stock Issued During Period, Value, New Issues					129,322	11,212
Proceeds from Issuance of Common Stock, Net					$ 129,322	$ 11,212
Stock Issued During Period, Shares, Warrants Exercised (in shares)					45,114,210
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)						(3,053,000)	(7,900,000)
Stock Issued During Period Shares Stock Options and Warrants Exercised (in shares)					0
Stock Issued During Period, Value, New Issues					$ 0	$ 0
Common Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)					70,666,000	7,030,000
Stock Issued During Period, Shares, Conversion of Convertible Securities (in shares)						3,053,000
Stock Issued During Period Shares Stock Options and Warrants Exercised (in shares)					45,114,000			4,900,000
Stock Issued During Period, Value, New Issues					$ 71	$ 7
Series C Warrants [Member]
Proceeds from Warrant Exercises								$ 3,300
Series B Warrant [Member]
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
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
Stock Offering Agreement, Commission, Percent												3.00%
Proceeds from Issuance or Sale of Equity, Total					28,100			$ 11,600	$ 7,200
Proceeds from Issuance of Equity Net					$ 27,257			$ 11,200	$ 7,000
Stock Issued During Period, Shares, New Issues (in shares)					19,120,037
Shares Issued, Price Per Share (in dollars per share)					$ 1.47
The 2020 ATM Offering [Member]
Stock Offering Agreement, Maximum Share Value										$ 40,000
Stock Offering Agreement, Commission, Percent										3.00%
Public Offering [Member]
Proceeds from Issuance of Equity Net			$ 13,600	$ 13,500
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
Proceeds from Issuance of Common Stock	$ 79,600				$ 73,400
Proceeds from Issuance of Common Stock, Net	$ 73,400
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
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
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
Proceeds from Issuance of Common Stock		$ 28,600			$ 28,600