Note 9 - Stockholders' Equity - Unvested Units Expected to Vest (Details) - shares	Dec. 31, 2017	Dec. 31, 2016
Units scheduled to vest (in shares)	2,095,318	2,735,564
Share-based Compensation Award, Tranche One [Member]
Units scheduled to vest (in shares)	669,756
Share-based Compensation Award, Tranche Two [Member]
Units scheduled to vest (in shares)	549,041
Share-based Compensation Award, Tranche Three [Member]
Units scheduled to vest (in shares)	622,036
Share-based Compensation Award, Tranche Four [Member]
Units scheduled to vest (in shares)	254,485