Note 6 - Note Payable (Details Textual) - USD ($)			3 Months Ended		6 Months Ended
	May 22, 2017	Jun. 20, 2016	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Class of Warrant or Right, Number of Securities Called by Warrants or Rights			642,622		642,622
Class of Warrant or Right, Exercise Price of Warrants or Rights
Interest Payable			$ 18,125,000		$ 18,125,000
June 2016 Issuance Related to 2016 Loan Agreement [Member]
Warrant Term		10 years
Class of Warrant or Right, Number of Securities Called by Warrants or Rights		100,402	100,402		100,402
Class of Warrant or Right, Exercise Price of Warrants or Rights		$ 4.98	$ 4.98		$ 4.98
May 2017 Issuance Related to 2017 Loan Agreement [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights	222,049		222,049		222,049
Class of Warrant or Right, Exercise Price of Warrants or Rights	$ 9.50		$ 9.50		$ 9.50
Warrants and Rights Outstanding, Term	10 years
The 2016 Loan Agreement [Member] | Western Alliance Bank [Member]
Debt Instrument, Face Amount		$ 10,000,000
Debt Instrument, Fee, Percentage		4.00%
The 2016 Loan Agreement [Member] | Western Alliance Bank [Member] | First Tranche [Member]
Proceeds from Issuance of Long-term Debt, Total		$ 7,500,000
The 2016 Loan Agreement [Member] | Western Alliance Bank [Member] | Second Tranche [Member]
Proceeds from Issuance of Long-term Debt, Total		$ 2,500,000
The 2017 Loan Agreement [Member] | CRG LP [Member]
Debt Instrument, Face Amount	$ 20,000,000
Debt Agreement, Covenant, Additional Funding	10,000,000
Debt Agreement, Maximum Borrowing Capacity	$ 30,000,000
Debt Instrument, Term	6 years
Debt instrument, Interest Only Payment, Period	4 years
Debt Instrument, Interest Rate, Stated Percentage	12.50%
Debt Instrument, Interest Rate, Stated Percentage Deferred During Interest-only Period	4.00%
Paid-in-Kind Interest			$ 311,000	$ 80,000	$ 616,000	$ 80,000
Debt Instrument, Prepayment Fee, Percentage	5.00%
Debt Instrument, Prepayment Fee, Period of Term	5 years
Debt Instrument, Covenant, Cash and Cash Equivalents	$ 2,000,000
The 2017 Loan Agreement [Member] | CRG LP [Member] | Other Noncurrent Liabilities [Member]
Interest Payable			$ 305,000		$ 305,000