Note 12 - Stock-based Compensation (Tables)

6 Months Ended

Jun. 30, 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		1,392		$	4.52
Cancelled		(230	)	$	7.17
Exercised		(225	)	$	5.33
Expired		(27	)	$	7.79
Outstanding at June 30, 2019		7,174		$	6.38		6.44
Exercisable at June 30, 2019		4,647		$	6.63		5.03
Vested and expected to vest at June 30, 2019		6,519		$	6.46		6.17

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

	Three Months Ended						Six Months Ended
	June 30,						June 30,
	2019			2018			2019			2018
Risk-free interest rate		2.04	%		2.91	%		2.61	%		2.71	%
Expected life (in years)		5.27			7.15			6.52			6.72
Expected volatility		60	%		70	%		66	%		69	%
Expected dividend yield		--			--			--			--

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

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2019		2018		2019		2018
Research and development expenses	$	387	$	452	$	837	$	977
Selling, general and administrative expenses		607		1,678		1,234		2,201
Total stock-based compensation expense	$	994	$	2,130	$	2,071	$	3,178