Note 10 - Convertible Preferred Stock and Stockholder's Equity - Warrants Outstanding to Acquire Shares of Its Common Stock (Details)	Dec. 31, 2017 $ / shares shares
Common stock warrants, outstanding (in shares) | shares	14,003,608
Convertible Preferred Stock Issued Prior to Merger [Member]
Common stock warrants, outstanding (in shares) | shares	447,721
Convertible Preferred Stock Issued Prior to Merger [Member] | Minimum [Member]
Range of exercise price (in dollars per share) | $ / shares	$ 20
Convertible Preferred Stock Issued Prior to Merger [Member] | Maximum [Member]
Range of exercise price (in dollars per share) | $ / shares	$ 49
Series A Convertible Preferred Stock [Member]
Common stock warrants, outstanding (in shares) | shares	13,555,887
Range of exercise price (in dollars per share) | $ / shares	$ 2.22