Note 12 - Share-based Compensation - Stock Options Outstanding (Details)
 3 Months Ended
Mar. 31, 2020
$ / shares
shares
Outstanding, shares (in shares) | shares 1,181,309
Outstanding, weighted average remaining contractual life (Year) 8 years
Outstanding, weighted average exercise price (in dollars per share) $ 5.21
Exercisable, shares (in shares) | shares 677,566
Exercisable, weighted average remaining contractual life (Year) 7 years 109 days
Exercise Price Range 1 [Member]  
Per share exercise price, lower range (in dollars per share) $ 2
Per share exercise price, upper range (in dollars per share) $ 2.99
Outstanding, shares (in shares) | shares 125,700
Outstanding, weighted average remaining contractual life (Year) 5 years 292 days
Outstanding, weighted average exercise price (in dollars per share) $ 2.16
Exercisable, shares (in shares) | shares 125,700
Exercisable, weighted average remaining contractual life (Year) 5 years 292 days
Exercise Price Range 2 [Member]  
Per share exercise price, lower range (in dollars per share) $ 3
Per share exercise price, upper range (in dollars per share) $ 3.99
Outstanding, shares (in shares) | shares 98,572
Outstanding, weighted average remaining contractual life (Year) 6 years 255 days
Outstanding, weighted average exercise price (in dollars per share) $ 3.68
Exercisable, shares (in shares) | shares 98,572
Exercisable, weighted average remaining contractual life (Year) 6 years 255 days
Exercise Price Range 3 [Member]  
Per share exercise price, lower range (in dollars per share) $ 4
Per share exercise price, upper range (in dollars per share) $ 4.99
Outstanding, shares (in shares) | shares 718,112
Outstanding, weighted average remaining contractual life (Year) 9 years
Outstanding, weighted average exercise price (in dollars per share) $ 4.52
Exercisable, shares (in shares) | shares 311,658
Exercisable, weighted average remaining contractual life (Year) 8 years 255 days
Exercise Price Range 4 [Member]  
Per share exercise price, lower range (in dollars per share) $ 5
Per share exercise price, upper range (in dollars per share) $ 10
Outstanding, shares (in shares) | shares 196,275
Outstanding, weighted average remaining contractual life (Year) 7 years 182 days
Outstanding, weighted average exercise price (in dollars per share) $ 7.59
Exercisable, shares (in shares) | shares 98,986
Exercisable, weighted average remaining contractual life (Year) 7 years 292 days
Exercise Price Range 5 [Member]  
Per share exercise price, lower range (in dollars per share) $ 10.01
Per share exercise price, upper range (in dollars per share) $ 34
Outstanding, shares (in shares) | shares 42,650
Outstanding, weighted average remaining contractual life (Year) 2 years 109 days
Outstanding, weighted average exercise price (in dollars per share) $ 18.49
Exercisable, shares (in shares) | shares 42,650
Exercisable, weighted average remaining contractual life (Year) 2 years 109 days