Note 9 - Common Stock (Details Textual)				1 Months Ended					3 Months Ended				9 Months Ended
	Aug. 16, 2019 USD ($)	Feb. 12, 2018 USD ($)	Nov. 08, 2017 USD ($)	Aug. 31, 2019 USD ($) shares	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) shares	Jun. 30, 2018 USD ($) shares	Feb. 28, 2018 USD ($) shares	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) shares	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) shares	May 22, 2017 USD ($) yr $ / shares shares
Stock Issued During Period, Value, Restricted Stock Award, Gross							$ 256,000		$ 14,000	$ 47,000	$ 67,000	$ 108,000
Proceeds from Issuance of Common Stock													$ 6,322,000	$ 33,407,000
Stock Issued During Period, Shares, Restricted Stock Award, Gross | shares							1,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares									5,549				5,549
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
Class Of Warrant Or Right, Number Of Securities Called By Warrants Or Rights Expired In Period | shares									859	0			859	0
The 2017 Loan Agreement [Member]
Debt Issuance Costs, Net, Total									$ 790,000				$ 790,000
Amortization of Debt Discount (Premium)									$ 92,000	$ 82,000			$ 270,000	$ 242,000
May 2017 Issuance Related to 2017 Loan Agreement [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares									2,220				2,220		2,220
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares									$ 950				$ 950		$ 950
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
Restricted Stock [Member]
Proceeds From Issuance of Common Stock, Net					$ 274
Stock Issued During Period, Value, Restricted Stock Award, Gross					$ 25,000
August 2019 ATM Facility [Member]
Aggregate Offering Price	$ 6,760,000			$ 6,760,000
Stock Issued During Period, Shares, New Issues | shares									1,004,171
Proceeds From Issuance of Common Stock, Net									$ 6,322,000
November 2017 ATM Facility [Member]
Aggregate Offering Price			$ 25,000,000	$ 25,000,000
Stock Issued During Period, Shares, New Issues | shares				3,364					0	37			0	2,771
Proceeds From Issuance of Common Stock, Net				$ 1,318,000					$ 11,000	$ 11,000			$ 1,194,000	$ 1,194,000
December 2018 Offering [Member]
Stock Issued During Period, Shares, New Issues | shares						147,285
Proceeds From Issuance of Common Stock, Net						$ 20,385,000
Proceeds from Issuance of Common Stock						$ 22,093,000
February 2018 Offering [Member]
Stock Issued During Period, Shares, New Issues | shares								115,000
Proceeds From Issuance of Common Stock, Net		$ 32,214,000						$ 32,214,000
Stock Issued During Period, Value, Restricted Stock Award, Gross		115,000
Proceeds from Issuance of Common Stock		$ 34,500,000						$ 34,500,000