Note 8 - Stock-based Compensation - Options Outstanding (Details)	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Options outstanding (in shares) | shares	1,097,960
Options outstanding, weighted average remaining contractual life (Year)	7 years 343 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 9.06
Options exercisable (in shares) | shares	919,073
Options exercisable, weighted average remaining contractual life (Year)	8 years 109 days
Exercise Price Range 1 [Member]
Per share exercise price, lower limit (in dollars per share)	$ 0.88
Per share exercise price, upper limit (in dollars per share)	$ 1.10
Options outstanding (in shares) | shares	38,360
Options outstanding, weighted average remaining contractual life (Year)	4 years 215 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 1.04
Options exercisable (in shares) | shares	38,360
Options exercisable, weighted average remaining contractual life (Year)	4 years 215 days
Exercise Price Range 2 [Member]
Per share exercise price, lower limit (in dollars per share)	$ 2.28
Per share exercise price, upper limit (in dollars per share)	$ 2.50
Options outstanding (in shares) | shares	42,000
Options outstanding, weighted average remaining contractual life (Year)	5 years 317 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 2.47
Options exercisable (in shares) | shares	42,000
Options exercisable, weighted average remaining contractual life (Year)	5 years 317 days
Exercise Price Range 3 [Member]
Options outstanding (in shares) | shares	214,000
Options outstanding, weighted average remaining contractual life (Year)	6 years 339 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 3.18
Options exercisable (in shares) | shares	214,000
Options exercisable, weighted average remaining contractual life (Year)	6 years 339 days
Exercise Price Range 4 [Member]
Per share exercise price, lower limit (in dollars per share)	$ 6.10
Per share exercise price, upper limit (in dollars per share)	$ 8.10
Options outstanding (in shares) | shares	364,000
Options outstanding, weighted average remaining contractual life (Year)	8 years 215 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 7.59
Options exercisable (in shares) | shares	270,000
Options exercisable, weighted average remaining contractual life (Year)	9 years 332 days
Exercise Price Range 5 [Member]
Per share exercise price, lower limit (in dollars per share)	$ 10
Per share exercise price, upper limit (in dollars per share)	$ 10.10
Options outstanding (in shares) | shares	54,000
Options outstanding, weighted average remaining contractual life (Year)	9 years 113 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 10
Options exercisable (in shares) | shares	52,000
Options exercisable, weighted average remaining contractual life (Year)	9 years 189 days
Exercise Price Rang 6 [Member]
Options outstanding (in shares) | shares	385,600
Options outstanding, weighted average remaining contractual life (Year)	8 years 94 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 15.10
Options exercisable (in shares) | shares	302,713
Options exercisable, weighted average remaining contractual life (Year)	8 years 160 days