Note 8 - Common Stock (Details Textual)				3 Months Ended		6 Months Ended
	Jun. 08, 2018 USD ($) $ / shares shares	Feb. 12, 2018 USD ($) $ / shares shares	Nov. 08, 2017 USD ($)	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) shares	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) shares	May 22, 2017 USD ($) yr $ / shares shares	Jun. 20, 2016 USD ($) $ / shares shares
Stock Issued During Period, Shares, Restricted Stock Award, Gross | shares	100,000
Shares Issued, Price Per Share | $ / shares	$ 2.56
Stock Issued During Period, Value, Restricted Stock Award, Gross	$ 256,000
Proceeds from Issuance of Common Stock						$ 33,396,000	$ 31,440,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares				642,622		642,622
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares
Debt Instrument, Unamortized Discount, Total				$ 1,388,000		$ 1,388,000
Class of Warrants or Rights, Exercises in Period | shares				0	0	0	0
Class of Warrant or Right Number of Securities Called by Warrants or Rights Cancelled In Period | shares				0	0	0	0
Allocated Share-based Compensation Expense, Total				$ 776,000	$ 455,000	$ 1,529,000	$ 827,000
The 2016 Loan Agreement [Member]
Debt Issuance Costs, Net, Total									$ 90,000
Amortization of Debt Discount (Premium)					336,000		371,000
The 2017 Loan Agreement [Member]
Debt Issuance Costs, Net, Total				786,000		786,000
Amortization of Debt Discount (Premium)				160,000
Debt Instrument, Unamortized Discount, Total				$ 1,388,000		$ 1,388,000
June 2016 Issuance Related to 2016 Loan Agreement [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares				100,402		100,402			100,402
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares				$ 4.98		$ 4.98			$ 4.98
Warrants and Rights Outstanding									$ 350,000
May 2017 Issuance Related to 2017 Loan Agreement [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares				222,049		222,049		222,049
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares				$ 9.50		$ 9.50		$ 9.50
Warrants and Rights Outstanding								$ 940,000
Warrants and Rights Outstanding, Term								10 years
Allocated Share-based Compensation Expense, Total				$ 0	$ 0	$ 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
February 2018 Offering [Member]
Stock Issued During Period, Shares, New Issues | shares		11,500,000
Share Price | $ / shares		$ 3
Proceeds from Issuance of Common Stock		$ 34,500,000
Proceeds From Issuance of Common Stock, Net		$ 32,213,000
November 2017 ATM Facility [Member]
Stock Issued During Period, Shares, New Issues | shares						332,778
Proceeds From Issuance of Common Stock, Net						$ 1,308,000
Aggregate Offering Price			$ 25,000,000
ATM Facility [Member]
Stock Issued During Period, Shares, New Issues | shares				65,252		273,529
Proceeds From Issuance of Common Stock, Net				$ 172,000		$ 1,183,000