Note 6 - Stock-based Compensation - Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2014
Performance Shares [Member]
Reserved for future issuance (in shares)	2,999,166	2,419,901
The 2014 Employee Stock Purchase Plan [Member]
Reserved for future issuance (in shares)	296,685	297,359	140,500
Reserved for future issuance (in shares)	3,295,851	2,717,260