Note 6 - Long-term Debt (Details Textual) - USD ($)				1 Months Ended	12 Months Ended
	Aug. 01, 2019	May 30, 2019	Dec. 16, 2013	Dec. 31, 2013	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Oct. 01, 2017	Mar. 02, 2017	Dec. 31, 2016	Sep. 30, 2016	Sep. 18, 2015
Repayments of Long-term Debt, Total					$ 3,470,000	$ 7,718,000	$ 3,514,000
Interest Expense, Total					2,535,000	2,217,000	3,316,000
Long-term Debt, Total					25,147,000
Amortization of Debt Discount (Premium)					712,000	613,000	1,265,000
Loan Agreement with Oxford Finance LLC [Member]
Debt Instrument, Face Amount		$ 25,000,000
Debt Instrument, Final Payment, Percentage of Aggregate Principal Amount		5.00%
Repayments of Long-term Debt, Total		$ 8,900,000
Fair Value of Contingent Put Option, Liability					400,000
Interest Expense, Total					1,900,000
Proceeds from Debt, Net of Issuance Costs and Repayment of Debt		$ 15,900,000
Debt Instrument, Interest Rate, Base Percentage		2.50%
Prepayment Charge, Percentage of Outstanding Balance, Before May 30,2020		2.00%
Prepayment Charge, Percentage of Outstanding Balance, After May 30, 2020, Before May 30, 2021		1.50%
Prepayment Charge, Percentage of Outstanding Balance, After May 30,2021		1.00%
Debt Instrument, Covenant, Minimum Required Unrestircted Cash		$ 5,000,000
Debt Instrument, Minimum Indebtedness Amount with Acceleration Right		250,000
Loan Default Events, Trigger of Negative Impact of Government Approvals and Judgments		$ 500,000
Debt Instrument, Default Additional Interest Rate		5.00%
Amortization of Debt Discount (Premium)					600,000
Loan Agreement with Oxford Finance LLC [Member] | Minimum [Member]
Net Unrestricted Cash Proceeds, Contingent Threshold for Interest Only Period of Loan		$ 45,000,000
Loan Agreement with Oxford Finance LLC [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate		6.75%
Hercules Loan and Security Agreement [Member]
Debt Instrument, Face Amount									$ 20,500,000
Debt Instrument, Final Payment, Percentage of Aggregate Principal Amount									6.50%
Repayments of Long-term Debt, Total		$ 8,900,000
Long-term Debt, Gross		7,400,000
Debt Instrument, Termination Fee		1,300,000
Gain (Loss) on Extinguishment of Debt, Total		$ (200,000)
Fair Value of Contingent Put Option, Liability						100,000
Long-term Line of Credit, Total						12,000,000
Interest Expense, Total					600,000	$ 2,200,000	$ 3,300,000
Hercules Loan and Security Agreement [Member] | First Tranche [Member]
Debt Instrument, Face Amount									$ 20,500,000
Amended and Restated Loan and Security Agreement [Member]
Long-term Debt Maturities Balloon Payments								$ 1,700,000
Debt Instrument, Variable Rate Threshold									9.55%
Debt Instrument, Percentage Used to Calculate Variable Rate									3.50%
Amended and Restated Loan and Security Agreement [Member] | Prime Rate [Member]
Debt Instrument, Interest Rate, Effective Percentage										9.55%
Site Readiness Agreement [Member]
Debt Instrument, Face Amount					$ 1,000,000
Debt Instrument, Interest Rate, Effective Percentage					13.61%
Long-term Debt, Gross					$ 1,500,000
Long-term Debt, Total					899,000
Debt Instrument, Periodic Payment, Principal	$ 500,000
Total Obligation Under Leasehold Agreement	$ 2,000,000
Hercules Warrants [Member]
Common Stock Warrants Exercised			176,730	176,730
Class of Warrant or Right, Exercise Price of Warrants or Rights			$ 6.79	$ 6.79							$ 3.07	$ 3.88
Warrant In Connection with Oxford Finance Loan Agreement [Member]
Class of Warrant or Right, Exercise Price of Warrants or Rights		$ 2.83
Class of Warrant or Right, Number of Securities Called by Warrants or Rights		176,679
Loan Agreement with Oxford Finance LLC [Member]
Long-term Debt, Total					$ 24,200,000