| Note 11 - Related Party Transactions (Details Textual) $ in Thousands | 6 Months Ended | 
|---|---|
| Jun. 30, 2019  USD ($) | |
| Moores Cancer Center at University of California San Diego [Member] | Research and Development Expense, Related Party [Member] | |
| Related Party Transaction, Amounts of Transaction | $ 135 | 
| X | ||||||||||
| 
- Definition Amount of transactions with related party during the financial reporting period. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
 | 
| X | ||||||||||
| 
- Details 
 | 
| X | ||||||||||
| 
- Details 
 |