Note 9 - Common Stock - Common Stock Reserved for Future Issuance (Details) - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Reserved for future issuance (in shares)	25,877,695	3,275,291
The 2014 Employee Stock Purchase Plan [Member]
Reserved for future issuance (in shares)	192,463	292,821	140,500
Performance Shares [Member]
Reserved for future issuance (in shares)	3,572,457	2,982,470
Conversion of Series A Preferred Stock to Common Stock [Member]
Reserved for future issuance (in shares)	22,112,775