Note 11 - Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

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, 2019		6,264		$	6.79		6.02
Granted		2,060		$	4.78
Cancelled		(921	)	$	6.35
Exercised		(386	)	$	5.03
Expired		(77	)	$	6.22
Outstanding at December 31, 2019		6,940		$	6.35		6.09
Exercisable at December 31, 2019		4,660		$	6.65		4.77
Vested and expected to vest at December 31, 2019		6,440		$	6.43		5.86

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

	2019		2018		2017
Risk-free interest rate		2.35%		2.72%		2.17%
Expected life (in years)		6.73		6.67		6.76
Expected volatility		66%		69%		72%
Expected dividend yield		--		--		--

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

	2019		2018		2017
Research and development expenses	$	1,770	$	1,760	$	1,371
Selling, general and administrative expenses		2,622		3,449		2,771
Total stock-based compensation expense	$	4,392	$	5,209	$	4,142