Note 10 - Stock-based Compensation - Stock Option Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Options outstanding, number (in shares)	170,461	147,214
Options outstanding, weighted-average exercise price (in dollars per share)	$ 109.74	$ 121.37
Granted (in shares)	44,005	23,345
Granted, weighted-average exercise price (in dollars per share)	$ 15.37	$ 36.92
Expired (in shares)	(147)	(98)
Expired, weighted-average exercise price (in dollars per share)	$ 256.50	$ 225
Options outstanding, weighted-average remaining contractual life (Year)	6 years 255 days	7 years 47 days
Options outstanding, aggregate intrinsic value	$ 143	$ 4,746
Forfeited (in shares)	(10,583)
Forfeited, weighted-average exercise price (in dollars per share)	$ 131.25
Options outstanding, number (in shares)	203,736	170,461
Options outstanding, weighted-average exercise price (in dollars per share)	$ 88.14	$ 109.74
Exercisable (in shares)	167,585
Exercisable, weighted-average exercise price (in dollars per share)	$ 101.69
Exercisable, weighted-average remaining contractual life (Year)	6 years 91 days
Exercisable, aggregate intrinsic value	$ 143
Vested and expected to vest (in shares)	203,736
Vested and expected to vest, weighted-average exercise price (in dollars per share)	$ 88.14
Vested and expected to vest, weighted-average remaining contractual life (Year)	6 years 255 days
Vested and expected to vest, aggregate intrinsic value	$ 143