Note 6 - Stock-based Compensation - Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2014
Reserved for future issuance (in shares)	27,291,765	25,877,695
The 2014 Employee Stock Purchase Plan [Member]
Reserved for future issuance (in shares)	140,479	192,463	140,500
Performance Shares [Member]
Reserved for future issuance (in shares)	5,038,511	3,572,457
Conversion of Series A Preferred Stock to Common Stock [Member]
Reserved for future issuance (in shares)	22,112,775	22,112,775