Note 1 - Basis of Presentation (Details Textual) - USD ($)		1 Months Ended	3 Months Ended							9 Months Ended		10 Months Ended	12 Months Ended
	Jan. 01, 2019	May 31, 2017	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 30, 2018
Net Income (Loss) Attributable to Parent, Total			$ (7,444,000)	$ (7,476,000)	$ (7,260,000)		$ (6,240,000)	$ (6,032,000)	$ (5,971,000)	$ (22,180,000)	$ (18,243,000)
Working Capital			13,300,000							13,300,000		$ 13,300,000
Net Cash Provided by (Used in) Operating Activities, Total										(10,704,000)	(8,644,000)
Proceeds from Issuance of Common Stock										14,890,000	$ 14,581,000
Required Payment to Acquire Property Available for Purchase, Due End of Fiscal Year			$ 18,000,000							$ 18,000,000		$ 18,000,000
Percentage of Future Quarterly Interest Payments			25.00%							25.00%		25.00%
Interest Payments in Stock										$ 1,000,000
Debt Conversion, Original Debt, Amount										$ 5,600,000
Stock Issued During Period, Shares, Conversion of Convertible Securities										845,081
Operating Lease, Liability, Total			$ 69,000							$ 69,000		$ 69,000
Current Period Reclassification Adjustment	$ 865,000				865,000	[1]
Additional Paid-in Capital [Member]
Net Income (Loss) Attributable to Parent, Total
Current Period Reclassification Adjustment				$ 24,000	$ 2,896,000	[1]
Accounting Standards Update 2016-02 [Member]
Operating Lease, Right-of-Use Asset	151,000
Operating Lease, Liability, Total	100,000
Reclassification of Warrant Liability to Additional Paid-in Capital [Member]
Current Period Reclassification Adjustment			24,000							865,000
Gains from Revaluating Warrants													$ 1,500,000	$ 500,000
Reclassification of Warrant Liability to Additional Paid-in Capital [Member] | Additional Paid-in Capital [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	2,000,000
Reclassification of Warrant Liability to Additional Paid-in Capital [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 2,000,000
Senior Secured Debt [Member]
Debt Instrument, Face Amount		$ 60,000,000
Debt Instrument, Additional Amount after Refinance		15,000,000
Convertible Debt, Option Agreements, Percentage				99.00%
Long-term Debt, Total			69,200,000							$ 69,200,000		69,200,000
Debt Instrument, Maturity Date, Percentage of Conversion Rate,										120.00%
Debt Instrument, Maturity Date, Minimum Principal Outstanding Amount			10,000,000							$ 10,000,000		10,000,000
Prior Senior Secured Debt [Member]
Extinguishment of Debt, Amount		$ 45,000,000
2020 Convertible Senior Notes [Member]
Long-term Debt, Maturities, Repayments of Principal and Interest in Year Two			$ 73,700,000							$ 73,700,000		$ 73,700,000
Debt Instrument, Interest Rate, Stated Percentage				7.00%
Debt Instrument, Convertible, Conversion Price				$ 6.75
At Market Issuance Sales Agreement [Member]
At Market Issuance Sales Agreement, Maximum Aggregate Offering Price															$ 25,000,000
Stock Issued During Period, Shares, New Issues												1,557,557
Proceeds from Issuance of Common Stock, Gross												$ 16,400,000
Proceeds from Issuance of Common Stock												$ 16,000,000

[1] A cumulative effect adjustment of $2,031 thousand was recognized as of January 1, 2019, upon adoption of ASU 2017-11.