Note 12 - 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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Total operating expenses	$ 6,866	$ 7,166	$ 7,854	$ 6,819	$ 6,187	$ 5,082	$ 4,981	$ 4,620	$ 28,705	$ 20,870	$ 10,528
Net loss attributable to common stockholders	$ (6,962)	$ (7,107)	$ (7,905)	$ (6,552)	$ (6,342)	$ (5,383)	$ (5,072)	$ (4,580)	$ (28,526)	$ (21,377)	$ (9,695)
Net loss per share attributable to common stockholders - basic and diluted (in dollars per share)	$ (0.42)	$ (0.43)	$ (0.48)	$ (0.40)	$ (0.38)	$ (0.33)	$ (0.31)	$ (0.28)	$ (1.72)	$ (1.30)	$ (3.16)
Weighted-average common shares outstanding used in net loss per share attributable to common stockholders - basic and diluted (in shares)	16,595,502	16,582,276	16,561,239	16,507,586	16,490,430	16,466,945	16,449,937	16,448,688	16,561,799	16,464,123	3,064,507