Note 9 - Stock Options and Stock-based Employee Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Stock-based compensation expense	$ 2,443	$ 1,689
Research and Development Expense [Member]
Stock-based compensation expense	939	714
Selling, General and Administrative Expenses [Member]
Stock-based compensation expense	$ 1,504	$ 975

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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