Note 9 - Stock-based Compensation (Details Textual) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Share-based Payment Arrangement, Noncash Expense, Total	$ 685	$ 1,028
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0
Proceeds from Stock Options Exercised	$ 0	$ 0
Share-based Payment Arrangement, Option [Member]
Share-based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount	$ 414
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	1 year 255 days
Restricted Stock Units (RSUs) [Member]
Share-based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount	$ 4,306
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	2 years 219 days

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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