Note 9 - Summary of Stock Options - Summary of Options Outstanding and Exercisable (Details)	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Options outstanding, number (in shares) | shares	2,489,979
Options outstanding, weighted average exercise price (in dollars per share)	$ 5.88
Options outstanding, weighted average remaining contractual term (Year)	8 years 255 days
Options exercisable, number exercisable (in shares) | shares	734,721
Options exercisable, weighted average exercise price (in dollars per share)	$ 5.90
Range One [Member]
Options outstanding, number (in shares) | shares	12,500
Options outstanding, weighted average exercise price (in dollars per share)	$ 2.64
Options outstanding, weighted average remaining contractual term (Year)	7 years 226 days
Options exercisable, number exercisable (in shares) | shares	7,553
Options exercisable, weighted average exercise price (in dollars per share)	$ 2.64
Range Two [Member]
Options outstanding, number (in shares) | shares	66,876
Options outstanding, weighted average exercise price (in dollars per share)	$ 3.76
Options outstanding, weighted average remaining contractual term (Year)	7 years 127 days
Options exercisable, number exercisable (in shares) | shares	43,191
Options exercisable, weighted average exercise price (in dollars per share)	$ 3.76
Exercise price range, lower limit (in dollars per share)	3.68
Exercise price range, upper limit (in dollars per share)	$ 3.76
Range Three [Member]
Options outstanding, number (in shares) | shares	400,329
Options outstanding, weighted average exercise price (in dollars per share)	$ 4.63
Options outstanding, weighted average remaining contractual term (Year)	8 years 91 days
Options exercisable, number exercisable (in shares) | shares	161,941
Options exercisable, weighted average exercise price (in dollars per share)	$ 4.77
Exercise price range, lower limit (in dollars per share)	4.46
Exercise price range, upper limit (in dollars per share)	$ 4.92
Range Four [Member]
Options outstanding, number (in shares) | shares	567,679
Options outstanding, weighted average exercise price (in dollars per share)	$ 5.22
Options outstanding, weighted average remaining contractual term (Year)	9 years 73 days
Options exercisable, number exercisable (in shares) | shares	108,555
Options exercisable, weighted average exercise price (in dollars per share)	$ 5.22
Range Five [Member]
Options outstanding, number (in shares) | shares	254,000
Options outstanding, weighted average exercise price (in dollars per share)	$ 5.62
Options outstanding, weighted average remaining contractual term (Year)	9 years 339 days
Options exercisable, number exercisable (in shares) | shares
Options exercisable, weighted average exercise price (in dollars per share)
Exercise price range, lower limit (in dollars per share)	5.58
Exercise price range, upper limit (in dollars per share)	$ 5.67
Range Six [Member]
Options outstanding, number (in shares) | shares	557,753
Options outstanding, weighted average exercise price (in dollars per share)	$ 6
Options outstanding, weighted average remaining contractual term (Year)	8 years 80 days
Options exercisable, number exercisable (in shares) | shares	244,023
Options exercisable, weighted average exercise price (in dollars per share)	$ 6
Range Seven [Member]
Options outstanding, number (in shares) | shares	150,639
Options outstanding, weighted average exercise price (in dollars per share)	$ 6.44
Options outstanding, weighted average remaining contractual term (Year)	8 years 76 days
Options exercisable, number exercisable (in shares) | shares	45,574
Options exercisable, weighted average exercise price (in dollars per share)	$ 6.34
Exercise price range, lower limit (in dollars per share)	6.24
Exercise price range, upper limit (in dollars per share)	$ 6.61
Range Eight [Member]
Options outstanding, number (in shares) | shares	441,825
Options outstanding, weighted average exercise price (in dollars per share)	$ 7.65
Options outstanding, weighted average remaining contractual term (Year)	9 years 36 days
Options exercisable, number exercisable (in shares) | shares	85,506
Options exercisable, weighted average exercise price (in dollars per share)	$ 7.72
Exercise price range, lower limit (in dollars per share)	7
Exercise price range, upper limit (in dollars per share)	$ 7.92
Range Nine [Member]
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)	5 years 51 days
Options exercisable, number exercisable (in shares) | shares	38,135
Options exercisable, weighted average exercise price (in dollars per share)	$ 9.92
Range Ten [Member]
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)	3 years 76 days
Options exercisable, number exercisable (in shares) | shares	243
Options exercisable, weighted average exercise price (in dollars per share)	$ 100.46
Exercise price range, lower limit (in dollars per share)	56
Exercise price range, upper limit (in dollars per share)	$ 296