Note 8 - Stock-based Compensation - Stock Option Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Options outstanding, number (in shares)	203,736	170,461
Options outstanding, weighted-average exercise price (in dollars per share)	$ 88.14	$ 109.74
Granted (in shares)	117,270	44,005
Granted, weighted-average exercise price (in dollars per share)	$ 2.62	$ 15.37
Forfeited (in shares)	(11,583)	(10,583)
Forfeited, weighted-average exercise price (in dollars per share)	$ 83.81	$ 131.25
Expired (in shares)	(147)	(147)
Expired, weighted-average exercise price (in dollars per share)	$ 276	$ 256.50
Options outstanding, weighted-average remaining contractual life (Year)	6 years 357 days	6 years 255 days
Options outstanding, aggregate intrinsic value		$ 143
Options outstanding, number (in shares)	309,276	203,736
Options outstanding, weighted-average exercise price (in dollars per share)	$ 55.78	$ 88.14
Options outstanding, weighted-average remaining contractual life (Year)	6 years 357 days	6 years 255 days
Exercisable (in shares)	235,415
Exercisable, weighted-average exercise price (in dollars per share)	$ 71.75
Exercisable, weighted-average remaining contractual life (Year)	6 years 124 days
Exercisable, aggregate intrinsic value
Vested and expected to vest (in shares)	309,276
Vested and expected to vest, weighted-average exercise price (in dollars per share)	$ 55.78
Vested and expected to vest, weighted-average remaining contractual life (Year)	6 years 357 days
Vested and expected to vest, aggregate intrinsic value