Note 7 - Stock-based Compensation - Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2014
Reserved for future issuance (in shares)	3,834,918	3,275,291
The 2014 Employee Stock Purchase Plan [Member]
Reserved for future issuance (in shares)	247,774	292,821	140,500
Performance Shares [Member]
Reserved for future issuance (in shares)	3,587,144	2,982,470