Note 10 - Summary of Stock Options - Summary of Options Outstanding and Exercisable (Details)	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Options outstanding, number (in shares) | shares	5,593,130
Options outstanding, weighted average exercise price (in dollars per share)	$ 3.28
Options outstanding, weighted average remaining contractual term (Year)	8 years 58 days
Options exercisable, number exercisable (in shares) | shares	1,725,352
Options exercisable, weighted average exercise price (in dollars per share)	$ 5.04
Range One [Member]
Exercise price range, lower limit (in dollars per share)	1
Exercise price range, upper limit (in dollars per share)	$ 1.97
Options outstanding, number (in shares) | shares	2,655,417
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.38
Options outstanding, weighted average remaining contractual term (Year)	9 years 244 days
Options exercisable, number exercisable (in shares) | shares	183,487
Options exercisable, weighted average exercise price (in dollars per share)	$ 1.51
Range Two [Member]
Exercise price range, lower limit (in dollars per share)	2.02
Exercise price range, upper limit (in dollars per share)	$ 2.83
Options outstanding, number (in shares) | shares	97,500
Options outstanding, weighted average exercise price (in dollars per share)	$ 2.42
Options outstanding, weighted average remaining contractual term (Year)	8 years 339 days
Options exercisable, number exercisable (in shares) | shares	12,240
Options exercisable, weighted average exercise price (in dollars per share)	$ 2.64
Range Three [Member]
Exercise price range, lower limit (in dollars per share)	3.03
Exercise price range, upper limit (in dollars per share)	$ 3.82
Options outstanding, number (in shares) | shares	484,376
Options outstanding, weighted average exercise price (in dollars per share)	$ 3.55
Options outstanding, weighted average remaining contractual term (Year)	8 years 295 days
Options exercisable, number exercisable (in shares) | shares	93,126
Options exercisable, weighted average exercise price (in dollars per share)	$ 3.77
Range Four [Member]
Exercise price range, lower limit (in dollars per share)	4.30
Exercise price range, upper limit (in dollars per share)	$ 4.97
Options outstanding, number (in shares) | shares	1,000,954
Options outstanding, weighted average exercise price (in dollars per share)	$ 4.52
Options outstanding, weighted average remaining contractual term (Year)	7 years 251 days
Options exercisable, number exercisable (in shares) | shares	437,550
Options exercisable, weighted average exercise price (in dollars per share)	$ 4.55
Range Five [Member]
Exercise price range, lower limit (in dollars per share)	5.01
Exercise price range, upper limit (in dollars per share)	$ 5.67
Options outstanding, number (in shares) | shares	630,739
Options outstanding, weighted average exercise price (in dollars per share)	$ 5.35
Options outstanding, weighted average remaining contractual term (Year)	5 years 357 days
Options exercisable, number exercisable (in shares) | shares	411,788
Options exercisable, weighted average exercise price (in dollars per share)	$ 5.35
Range Six [Member]
Exercise price range, lower limit (in dollars per share)	6
Exercise price range, upper limit (in dollars per share)	$ 6.61
Options outstanding, number (in shares) | shares	463,483
Options outstanding, weighted average exercise price (in dollars per share)	$ 6.02
Options outstanding, weighted average remaining contractual term (Year)	3 years 171 days
Options exercisable, number exercisable (in shares) | shares	415,932
Options exercisable, weighted average exercise price (in dollars per share)	$ 6.02
Range Seven [Member]
Exercise price range, lower limit (in dollars per share)	7
Exercise price range, upper limit (in dollars per share)	$ 7.92
Options outstanding, number (in shares) | shares	250,227
Options outstanding, weighted average exercise price (in dollars per share)	$ 7.68
Options outstanding, weighted average remaining contractual term (Year)	6 years 339 days
Options exercisable, number exercisable (in shares) | shares	160,795
Options exercisable, weighted average exercise price (in dollars per share)	$ 7.68
Range Eight [Member]
Exercise price range, lower limit (in dollars per share)	9.92
Exercise price range, upper limit (in dollars per share)	$ 9.92
Options outstanding, number (in shares) | shares	10,424
Options outstanding, weighted average exercise price (in dollars per share)	$ 9.92
Options outstanding, weighted average remaining contractual term (Year)	3 years 310 days
Options exercisable, number exercisable (in shares) | shares	10,424
Options exercisable, weighted average exercise price (in dollars per share)	$ 9.92
Range Nine [Member]
Exercise price range, lower limit (in dollars per share)	59.60
Exercise price range, upper limit (in dollars per share)	$ 149.04
Options outstanding, number (in shares) | shares	10
Options outstanding, weighted average exercise price (in dollars per share)	$ 149.04
Options outstanding, weighted average remaining contractual term (Year)	2 years 204 days
Options exercisable, number exercisable (in shares) | shares	10
Options exercisable, weighted average exercise price (in dollars per share)	$ 149.04