Note 9 - Common Stock (Details Textual)				1 Months Ended			3 Months Ended			6 Months Ended		20 Months Ended
	Jun. 08, 2018 USD ($) $ / shares shares	Feb. 12, 2018 USD ($) $ / shares shares	Nov. 08, 2017 USD ($)	Mar. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Feb. 28, 2018 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) shares	Mar. 31, 2018 USD ($)	Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) shares	Jun. 30, 2019 USD ($) $ / shares shares	May 22, 2017 USD ($) yr $ / shares shares
Stock Issued During Period, Shares, Restricted Stock Award, Gross | shares	100,000			27,473
Shares Issued, Price Per Share | $ / shares	$ 2.56			$ 0.91
Stock Issued During Period, Value, Restricted Stock Award, Gross	$ 256,000			$ 25,000				$ 67,000	$ 108,000
Proceeds from Issuance of Common Stock											$ 33,396,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares							642,622			642,622		642,622
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
The 2017 Loan Agreement [Member]
Debt Issuance Costs, Net, Total							$ 790,000			$ 790,000		$ 790,000
Amortization of Debt Discount (Premium)							$ 119,000	$ 83,000		$ 202,000	$ 160,000
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	222,049
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares							$ 9.50			$ 9.50		$ 9.50	$ 9.50
Warrants and Rights Outstanding, Term													10 years
Warrants and Rights Outstanding													$ 940,000
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
December 2018 Offering [Member]
Shares Issued, Price Per Share | $ / shares					$ 1.50
Stock Issued During Period, Shares, New Issues | shares					14,728,504
Proceeds from Issuance of Common Stock					$ 22,093,000
Proceeds From Issuance of Common Stock, Net					$ 20,385,000
February 2018 Offering [Member]
Shares Issued, Price Per Share | $ / shares		$ 3				$ 3
Stock Issued During Period, Shares, New Issues | shares		11,500,000				11,500,000
Proceeds from Issuance of Common Stock		$ 34,500,000				$ 34,500,000
Proceeds From Issuance of Common Stock, Net		$ 32,214,000				$ 32,214,000
November 2017 ATM Facility [Member]
Stock Issued During Period, Shares, New Issues | shares							0	65,252		0	273,529	336,498
Proceeds From Issuance of Common Stock, Net								$ 172,000			$ 1,183,000	$ 1,318,000
Aggregate Offering Price			$ 25,000,000