Note 12 - Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2020

Notes Tables

Share-based Payment Arrangement, Option, Activity [Table Text Block]

						Weighted
				Weighted		Average
	Number			Average		Remaining
	of Shares			Exercise Price		Contractual Life
Outstanding at January 1, 2020		6,940		$	6.35		6.09
Granted		1,233		$	4.77
Cancelled		(1,486	)	$	6.54
Exercised		(36	)	$	4.52
Outstanding at March 31, 2020		6,651		$	6.03		7.03
Exercisable at March 31, 2020		4,136		$	6.61		5.65
Vested and expected to vest at March 31, 2020		5,822		$	6.17		6.69

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	Three Months Ended
	March 31,
	2020		2019
Risk-free interest rate		0.94%		2.62%
Expected life (in years)		6.07		6.54
Expected volatility		61%		66%
Expected dividend yield		--		--

Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block]

	Three Months Ended
	March 31,
	2020		2019
Research and development expenses	$	487	$	450
Selling, general and administrative expenses		391		627
Total stock-based compensation expense	$	878	$	1,077