Note 12 - Equity-based Compensation - Weighted-average Assumptions Used in Determining the Value of Options Granted to Non-employees (Details) - Nonemployees [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Expected price volatility	87.41%	87.68%	83.77%
Expected term (Year)	10 years	10 years	9 years 219 days
Risk-free interest rate	2.27%	1.61%	2.18%
Dividend yield	0.00%	0.00%	0.00%
Weighted-average fair value of options granted during the period (in dollars per share)	$ 2.40	$ 2.29	$ 7.15