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Note 8 - Stockholders' Equity - Common Stock Reserved for Future Issuance (Details) - shares
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Stock options outstanding (in shares) 13,462,963 11,845,376 8,434,988 7,918,004
Stock options available for grant (in shares) 4,685,432      
Employee stock purchase plan (in shares) 267,317      
Warrants outstanding (in shares) 620,164      
Shares of common stock underlying Convertible Notes outstanding (see Note 7) (in shares) 7,982,957      
Total shares reserved for future issuance (in shares) 27,018,833