Note 9 - Summary of Stock Options - Summary of Options Outstanding and Exercisable (Details)	6 Months Ended
	Jun. 30, 2018 $ / shares shares
Exercise price range, lower limit (in dollars per share)
Exercise price range, upper limit (in dollars per share)	$ 2.64
Options outstanding, number (in shares) | shares	4,209,355
Options outstanding, weighted average exercise price (in dollars per share)	$ 4.76
Options outstanding, weighted average remaining contractual term (Year)	8 years 292 days
Options exercisable, number exercisable (in shares) | shares	1,124,400
Options exercisable, weighted average exercise price (in dollars per share)	$ 5.65
Range One [Member]
Exercise price range, lower limit (in dollars per share)	1.75
Exercise price range, upper limit (in dollars per share)	$ 1.97
Options outstanding, number (in shares) | shares	552,500
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.94
Options outstanding, weighted average remaining contractual term (Year)	9 years 328 days
Options exercisable, number exercisable (in shares) | shares	12,500
Options exercisable, weighted average exercise price (in dollars per share)	$ 1.97
Range Two [Member]
Exercise price range, lower limit (in dollars per share)	2.64
Exercise price range, upper limit (in dollars per share)	$ 2.72
Options outstanding, number (in shares) | shares	82,500
Options outstanding, weighted average exercise price (in dollars per share)	$ 2.67
Options outstanding, weighted average remaining contractual term (Year)	9 years 182 days
Options exercisable, number exercisable (in shares) | shares	9,896
Options exercisable, weighted average exercise price (in dollars per share)	$ 2.64
Range Three [Member]
Exercise price range, lower limit (in dollars per share)	3.11
Exercise price range, upper limit (in dollars per share)	$ 3.82
Options outstanding, number (in shares) | shares	319,376
Options outstanding, weighted average exercise price (in dollars per share)	$ 3.63
Options outstanding, weighted average remaining contractual term (Year)	9 years 73 days
Options exercisable, number exercisable (in shares) | shares	55,730
Options exercisable, weighted average exercise price (in dollars per share)	$ 3.76
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	1,477,533
Options outstanding, weighted average exercise price (in dollars per share)	$ 4.52
Options outstanding, weighted average remaining contractual term (Year)	9 years
Options exercisable, number exercisable (in shares) | shares	336,424
Options exercisable, weighted average exercise price (in dollars per share)	$ 4.65
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	822,652
Options outstanding, weighted average exercise price (in dollars per share)	$ 5.34
Options outstanding, weighted average remaining contractual term (Year)	8 years 255 days
Options exercisable, number exercisable (in shares) | shares	195,456
Options exercisable, weighted average exercise price (in dollars per share)	$ 5.22
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	567,878
Options outstanding, weighted average exercise price (in dollars per share)	$ 6.05
Options outstanding, weighted average remaining contractual term (Year)	7 years 219 days
Options exercisable, number exercisable (in shares) | shares	326,724
Options exercisable, weighted average exercise price (in dollars per share)	$ 6
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	348,538
Options outstanding, weighted average exercise price (in dollars per share)	$ 7.65
Options outstanding, weighted average remaining contractual term (Year)	8 years 73 days
Options exercisable, number exercisable (in shares) | shares	149,292
Options exercisable, weighted average exercise price (in dollars per share)	$ 7.68
Range Eight [Member]
Exercise price range, upper limit (in dollars per share)
Options outstanding, number (in shares) | shares	38,135
Options outstanding, weighted average exercise price (in dollars per share)	$ 9.92
Options outstanding, weighted average remaining contractual term (Year)	4 years 146 days
Options exercisable, number exercisable (in shares) | shares	38,135
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	243
Options outstanding, weighted average exercise price (in dollars per share)	$ 100.46
Options outstanding, weighted average remaining contractual term (Year)	2 years 182 days
Options exercisable, number exercisable (in shares) | shares	243
Options exercisable, weighted average exercise price (in dollars per share)	$ 100.46