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Note 7 - Stock-Based Compensation - Stock-based Compensation Allocation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Payment Arrangement, Expense $ 17,719 $ 9,396 $ 12,621
Research and Development Expense [Member]      
Share-based Payment Arrangement, Expense 13,977 6,867 9,602
General and Administrative Expense [Member]      
Share-based Payment Arrangement, Expense $ 3,742 $ 2,529 $ 3,019