Note 7 - Stock-based Compensation - Common Stock Reserved for Future Issuance (Details) - shares	Mar. 31, 2019	Dec. 31, 2018
Reserved for future issuance (in shares)	28,707,366	27,394,852
The 2014 Employee Stock Purchase Plan [Member]
Reserved for future issuance (in shares)	118,120	118,120
Performance Shares [Member]
Reserved for future issuance (in shares)	6,818,214	5,163,957
Conversion of Series A Preferred Stock to Common Stock [Member]
Reserved for future issuance (in shares)	21,771,032	22,112,775