Note 9 - Summary of Stock Options - Summary of Options Outstanding and Exercisable (Details)	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Options outstanding, number (in shares) | shares	3,690,902
Options outstanding, weighted average exercise price (in dollars per share)	$ 5.34
Options outstanding, weighted average remaining contractual term (Year)	8 years 229 days
Options exercisable, number exercisable (in shares) | shares	1,006,199
Options exercisable, weighted average exercise price (in dollars per share)	$ 5.74
Range One [Member]
Exercise price range, lower limit (in dollars per share)	2.64
Exercise price range, upper limit (in dollars per share)	$ 2.64
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 43 days
Options exercisable, number exercisable (in shares) | shares	9,115
Options exercisable, weighted average exercise price (in dollars per share)	$ 2.64
Range Two [Member]
Exercise price range, lower limit (in dollars per share)	3.66
Exercise price range, upper limit (in dollars per share)	$ 3.82
Options outstanding, number (in shares) | shares	171,876
Options outstanding, weighted average exercise price (in dollars per share)	$ 3.79
Options outstanding, weighted average remaining contractual term (Year)	8 years 281 days
Options exercisable, number exercisable (in shares) | shares	51,551
Options exercisable, weighted average exercise price (in dollars per share)	$ 3.76
Range Three [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	1,549,550
Options outstanding, weighted average exercise price (in dollars per share)	$ 4.52
Options outstanding, weighted average remaining contractual term (Year)	9 years 80 days
Options exercisable, number exercisable (in shares) | shares	270,103
Options exercisable, weighted average exercise price (in dollars per share)	$ 4.68
Range Four [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	912,800
Options outstanding, weighted average exercise price (in dollars per share)	$ 5.36
Options outstanding, weighted average remaining contractual term (Year)	8 years 339 days
Options exercisable, number exercisable (in shares) | shares	178,340
Options exercisable, weighted average exercise price (in dollars per share)	$ 5.22
Range Five [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	619,062
Options outstanding, weighted average exercise price (in dollars per share)	$ 6.06
Options outstanding, weighted average remaining contractual term (Year)	7 years 76 days
Options exercisable, number exercisable (in shares) | shares	345,227
Options exercisable, weighted average exercise price (in dollars per share)	$ 6.03
Range Six [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	386,736
Options outstanding, weighted average exercise price (in dollars per share)	$ 7.64
Options outstanding, weighted average remaining contractual term (Year)	8 years 69 days
Options exercisable, number exercisable (in shares) | shares	113,485
Options exercisable, weighted average exercise price (in dollars per share)	$ 7.71
Range Seven [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	38,135
Options outstanding, weighted average exercise price (in dollars per share)	$ 9.92
Options outstanding, weighted average remaining contractual term (Year)	4 years 237 days
Options exercisable, number exercisable (in shares) | shares	38,135
Options exercisable, weighted average exercise price (in dollars per share)	$ 9.92
Range Eight [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 262 days
Options exercisable, number exercisable (in shares) | shares	243
Options exercisable, weighted average exercise price (in dollars per share)	$ 100.46