Note 8 - Net Loss Per Share Attributable to Common Stockholders - Common Stock Equivalents Excluded from Calculation of Net Loss Per Share (Details) - shares	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Outstanding stock options (in shares)	2,843,335	2,212,816
Employee Stock Option [Member]
Outstanding stock options (in shares)	2,751,595	2,203,975
Outstanding ESPP Shares [Member]
Outstanding stock options (in shares)	91,740	8,841

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Name:	us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration