Note 10 - Stockholders' Equity - Unvested Units Expected to Vest (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Units scheduled to vest (in shares)	2,689,468	2,095,318
Share-based Compensation Award, Tranche One [Member]
Units scheduled to vest (in shares)	1,246,291
Share-based Compensation Award, Tranche Two [Member]
Units scheduled to vest (in shares)	866,680
Share-based Compensation Award, Tranche Three [Member]
Units scheduled to vest (in shares)	576,497