Note 13 - Quarterly Financial Information (Unaudited, in Thousands, Except Share and Per Share Data) - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017		Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total operating expenses	$ 5,590	$ 12,306		$ 5,986	$ 6,480	$ 6,866	$ 7,166	$ 7,854	$ 6,819	$ 30,362	$ 28,705	$ 20,870
Net loss attributable to common stockholders	$ (5,551)	$ (19,054)		$ (5,608)	$ (6,498)	$ (6,962)	$ (7,107)	$ (7,905)	$ (6,552)
Net loss per share attributable to common stockholders - basic and diluted (in dollars per share)	$ (0.33)	$ (1.08)		$ (0.33)	$ (0.39)	$ (0.42)	$ (0.43)	$ (0.48)	$ (0.40)	$ (2.13)	$ (1.72)	$ (1.30)
Weighted-average common shares outstanding used in net loss per share attributable to common stockholders - basic and diluted (in shares)	17,619,418	17,619,418	[1]	17,207,672	16,636,201	16,595,502	16,582,276	16,561,239	16,507,586	17,274,326	16,561,799	16,464,123

[1] Adjusted to correct an immaterial error in the weighted-average share calculation in the Company's Form 10-Q as of September 30, 2017.