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Note 9 - Stockholders' Equity - Stock-based Compensation Expense (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Stock-based compensation expense $ 17,902 $ 7,701
Research and Development Expense [Member]    
Stock-based compensation expense 5,324 3,036
General and Administrative Expense [Member]    
Stock-based compensation expense 3,680 2,276
Selling and Marketing Expense [Member]    
Stock-based compensation expense $ 8,898 $ 2,389