Note 11 - Stock-based Compensation - Stock Options Outstanding (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	May 16, 2017	May 15, 2017	Dec. 31, 2016	Dec. 31, 2015
Options outstanding (in shares)	703,442	200,000	2,400,000	211,213	154,581
Options outstanding, weighted average exercise price (in dollars per share)	$ 10.34			$ 61.59	$ 80.44
Options exercisable (in shares)	405,075
Exercisable (Year)	8 years 219 days
Range 1 [Member]
Range of exercise prices, upper range (in dollars per share)	$ 5
Options outstanding (in shares)	535,964
Options outstanding, weighted average remaining contractual term (Year)	9 years 146 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 2.69
Options exercisable (in shares)	256,720
Exercisable (Year)	9 years 146 days
Options exercisable, weighted average exercise price (in dollars per share)	$ 2.69
Range 2 [Member]
Range of exercise prices, upper range (in dollars per share)	$ 35
Options outstanding (in shares)	105,674
Options outstanding, weighted average remaining contractual term (Year)	7 years 328 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 24.64
Options exercisable (in shares)	88,153
Exercisable (Year)	7 years 328 days
Options exercisable, weighted average exercise price (in dollars per share)	$ 25.12
Range of exercise prices, lower range (in dollars per share)	$ 5
Range 3 [Member]
Options outstanding (in shares)	61,804
Options outstanding, weighted average remaining contractual term (Year)	6 years 73 days
Options outstanding, weighted average exercise price (in dollars per share)	$ 52.23
Options exercisable (in shares)	60,202
Exercisable (Year)	6 years 73 days
Options exercisable, weighted average exercise price (in dollars per share)	$ 52.32
Range of exercise prices, lower range (in dollars per share)	$ 35