Note 9 - Stockholders' Equity - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-based compensation expense	$ 33,367	$ 30,538	$ 25,956
Research and Development Expense [Member]
Stock-based compensation expense	13,689	11,312	11,316
General and Administrative Expense [Member]
Stock-based compensation expense	9,630	9,469	7,402
Selling and Marketing Expense [Member]
Stock-based compensation expense	$ 10,048	$ 9,757	$ 7,238

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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