Note 11 - Common Stock (Details Textual)				1 Months Ended								11 Months Ended	12 Months Ended
	Aug. 16, 2019 USD ($)	Feb. 12, 2018 USD ($)	Nov. 08, 2017 USD ($)	Dec. 31, 2019 USD ($) $ / shares shares	Nov. 30, 2019 USD ($) $ / shares shares	Oct. 31, 2019 USD ($)	Aug. 31, 2019 USD ($) shares	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares	Feb. 28, 2018 USD ($) shares	Nov. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Nov. 26, 2019 USD ($)	Nov. 12, 2019 USD ($) $ / shares shares	Sep. 30, 2019 USD ($)	May 22, 2017 USD ($) yr $ / shares shares
Shares Issued, Price Per Share | $ / shares										$ 2.56
Class of Warrant or Right, Number of Securities Called by Warrants or Rights				24,737,618					6,408				24,737,618	6,408
Stock Issued During Period, Value, Restricted Stock Award, Gross | $										$ 256,000			$ 74,000	$ 336,000
Proceeds from Issuance of Common Stock | $													16,274,000	53,792,000
Stock Issued During Period, Shares, Restricted Stock Award, Gross										1,000
Amortization of Debt Discount (Premium) | $													352,000	$ 325,000
Debt Instrument, Unamortized Discount, Total | $				$ 12,000									$ 12,000
Class of Warrants or Rights, Exercises in Period													0	0
Class of Warrant or Right Number of Securities Called by Warrants or Rights Cancelled In Period													0	0
Class Of Warrant Or Right, Number Of Securities Called By Warrants Or Rights Expired In Period													1,272	0
Share-based Payment Arrangement, Expense | $													$ 2,400,000	$ 3,035,000
The 2017 Loan Agreement [Member]
Debt Issuance Costs, Net, Total | $																	$ 790,000
Restricted Stock [Member]
Proceeds From Issuance of Common Stock, Net | $				30,675		$ 7,740		$ 274
Stock Issued During Period, Value, Restricted Stock Award, Gross | $				$ 25,000		$ 25,000		$ 25,000
Series A Convertible Preferred Stock Into Common Stock [Member]
Conversion of Stock, Shares Converted				3,021,237	600,000
Series B Convertible Preferred Stock Into Common Stock [Member] | Minimum [Member]
Conversion of Stock, Conversion Rate | $ / shares																$ 1.53
Series B Convertible Preferred Stock Into Common Stock [Member] | Maximum [Member]
Conversion of Stock, Conversion Rate | $ / shares																$ 1.836
Series A Preferred Stock [Member]
Convertible Preferred Stock, Shares Issued upon Conversion				1	1							1	1
Series B Preferred Stock [Member]
Conversion of Stock, Conversion Rate | $ / shares				$ 1.53									$ 1.53
Series A Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right					1							1
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares				$ 1.55	$ 1.55							$ 1.55	$ 1.55
Warrants and Rights Outstanding, Term					1 year							1 year
Class of Warrant or Right, Number of Securities Called by Warrants or Rights				7,419,353	7,419,353							7,419,353	7,419,353
Warrants and Rights Outstanding | $					$ 1,210,000							$ 1,210,000
Series A Warrants [Member] | Measurement Input, Expected Dividend Rate [Member]
Warrants and Rights Outstanding, Measurement Input					0							0
Series A Warrants [Member] | Measurement Input, Price Volatility [Member]
Warrants and Rights Outstanding, Measurement Input					91.1							91.1
Series A Warrants [Member] | Measurement Input, Risk Free Interest Rate [Member]
Warrants and Rights Outstanding, Measurement Input					1.59							1.59
Series B Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right					1							1
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares				$ 1.55	$ 1.55							$ 1.55	$ 1.55
Warrants and Rights Outstanding, Term					5 years							5 years
Class of Warrant or Right, Number of Securities Called by Warrants or Rights				7,419,353	7,419,353							7,419,353	7,419,353
Warrants and Rights Outstanding | $					$ 2,871,000							$ 2,871,000
Series B Warrants [Member] | Measurement Input, Expected Dividend Rate [Member]
Warrants and Rights Outstanding, Measurement Input					0							0
Series B Warrants [Member] | Measurement Input, Price Volatility [Member]
Warrants and Rights Outstanding, Measurement Input					68.8							68.8
Series B Warrants [Member] | Measurement Input, Risk Free Interest Rate [Member]
Warrants and Rights Outstanding, Measurement Input					1.58							1.58
Series A and Series B Warrants [Member]
Warrants and Rights Outstanding | $					$ 4,081,000							$ 4,081,000
Warrants Issued Upon Conversion of Term Loan with CRG [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares				$ 1.84									$ 1.84
Warrants and Rights Outstanding, Term																5 years
Class of Warrant or Right, Number of Securities Called by Warrants or Rights				9,893,776									9,893,776			9,893,776
Warrants and Rights Outstanding | $															$ 3,502,000	$ 3,502,000
Class of Warrants or Rights, Outstanding, Percentage of Common Stock																15.00%
Class of Warrants and Rights Outstanding, Exercise Price Percentage of Conversion Rate																120.00%
Warrants Issued Upon Conversion of Term Loan with CRG [Member] | Measurement Input, Expected Dividend Rate [Member]
Warrants and Rights Outstanding, Measurement Input																0
Warrants Issued Upon Conversion of Term Loan with CRG [Member] | Measurement Input, Price Volatility [Member]
Warrants and Rights Outstanding, Measurement Input																68.8
Warrants Issued Upon Conversion of Term Loan with CRG [Member] | Measurement Input, Risk Free Interest Rate [Member]
Warrants and Rights Outstanding, Measurement Input																1.58
May 2017 Issuance Related to 2017 Loan Agreement [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares				$ 950					$ 950				$ 950	$ 950				$ 950
Warrants and Rights Outstanding, Term																		10 years
Class of Warrant or Right, Number of Securities Called by Warrants or Rights				2,220					2,220				2,220	2,220				2,220
Warrants and Rights Outstanding | $																		$ 940,000
Share-based Payment Arrangement, Expense | $													$ 0	$ 0
May 2017 Issuance Related to 2017 Loan Agreement [Member] | Measurement Input, Expected Dividend Rate [Member]
Warrants and Rights Outstanding, Measurement Input																		0
May 2017 Issuance Related to 2017 Loan Agreement [Member] | Measurement Input, Price Volatility [Member]
Warrants and Rights Outstanding, Measurement Input																		0.551
May 2017 Issuance Related to 2017 Loan Agreement [Member] | Measurement Input, Risk Free Interest Rate [Member]
Warrants and Rights Outstanding, Measurement Input																		0.0225
May 2017 Issuance Related to 2017 Loan Agreement [Member] | Measurement Input, Expected Term [Member]
Warrants and Rights Outstanding, Measurement Input | yr																		10
Class A Units [Member] | Series A Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right					1							1
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares					$ 1.55							$ 1.55
Warrants and Rights Outstanding, Term					1 year							1 year
Class A Units [Member] | Series B Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right					1							1
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares					$ 1.55							$ 1.55
Warrants and Rights Outstanding, Term					5 years							5 years
Class B Units [Member] | Series A Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right					1							1
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares					$ 1.55							$ 1.55
Warrants and Rights Outstanding, Term					1 year							1 year
Class B Units [Member] | Series B Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right					1							1
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares					$ 1.55							$ 1.55
Warrants and Rights Outstanding, Term					5 years							5 years
Common Stock [Member]
Stock Issued During Period, Value, Restricted Stock Award, Gross | $
Stock Issued During Period, Shares, Restricted Stock Award, Gross													1,169	1,285
November 2019 Offering [Member]
Proceeds from Issuance or Sale of Equity, Total | $					$ 11,500,000
Proceeds From Issuance of Common Stock, Net | $					$ 9,952,000
November 2019 Offering [Member] | Series A Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues					5,473,410
November 2019 Offering [Member] | Series B Preferred Stock [Member]
Stock Issued During Period, Shares, New Issues												5,473,410
November 2019 Offering [Member] | Class A Units [Member]
Shares Issued, Price Per Share | $ / shares					$ 1.55							$ 1.55
Common Shares Per Unit					1							1
November 2019 Offering [Member] | Class B Units [Member]
Shares Issued, Price Per Share | $ / shares					$ 1.55							$ 1.55
Series A Convertible Preferred Stock Per Unit					1							1
November 2019 Offering [Member] | Class B Units [Member] | Series A Preferred Stock [Member]
Convertible Preferred Stock, Shares Issued upon Conversion					1							1
November 2019 Offering [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues					1,945,943							1,945,943	1,945,943
August 2019 ATM Facility [Member]
Proceeds From Issuance of Common Stock, Net | $													$ 6,322,000
Stock Issued During Period, Shares, New Issues													1,004,171
Aggregate Offering Price | $	$ 6,760,000						$ 6,760,000
August 2019 ATM Facility [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues													1,004,171
November 2017 ATM Facility [Member]
Proceeds From Issuance of Common Stock, Net | $					$ 1,193,000		$ 1,318,000							$ 1,193,000
Stock Issued During Period, Shares, New Issues					2,771		3,364						0	2,771
Aggregate Offering Price | $			$ 25,000,000				$ 25,000,000
November 2017 ATM Facility [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues														2,771
December 2018 Offering [Member]
Proceeds From Issuance of Common Stock, Net | $									$ 20,384,000
Stock Issued During Period, Shares, New Issues									147,285
Proceeds from Issuance of Common Stock | $									$ 22,093,000
December 2018 Offering [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues														147,285
February 2018 Offering [Member]
Proceeds From Issuance of Common Stock, Net | $		$ 32,214,000									$ 32,214,000
Stock Issued During Period, Shares, New Issues											115,000
Stock Issued During Period, Value, Restricted Stock Award, Gross | $		115,000
Proceeds from Issuance of Common Stock | $		$ 34,500,000									$ 34,500,000
February 2018 Offering [Member] | Common Stock [Member]
Stock Issued During Period, Shares, New Issues														115,000