Note 6 - Stock-based Compensation - Stock Option Activity (Details)	6 Months Ended
	Jun. 30, 2019 $ / shares shares
Options outstanding, number (in shares) | shares	203,736
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 88.14
Granted (in shares) | shares	117,270
Granted, weighted-average exercise price (in dollars per share) | $ / shares	$ 2.62
Forfeited (in shares) | shares	(11,583)
Forfeited, weighted-average exercise price (in dollars per share) | $ / shares	$ 83.81
Expired (in shares) | shares	(147)
Expired, weighted-average exercise price (in dollars per share) | $ / shares	$ 276
Options outstanding, number (in shares) | shares	309,276
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 55.78
Options outstanding, weighted-average remaining contractual life (Year)	7 years 175 days
Exercisable (in shares) | shares	187,910
Exercisable, weighted-average exercise price (in dollars per share) | $ / shares	$ 87.96
Exercisable, weighted-average remaining contractual life (Year)	6 years 62 days