Note 11 - Stock-based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Notes Tables

Share-based Compensation, Stock Options, Activity [Table Text Block]

						Weighted
				Weighted		Average
	Number			Average		Remaining
	of Shares			Exercise Price		Contractual Life
Outstanding at January 1, 2018		5,535		$	7.25		6.04
Granted		1,552		$	6.94
Exercised		(96	)	$	4.85
Cancelled		(331	)	$	8.06
Expired		(255	)	$	16.07
Outstanding at September 30, 2018		6,405		$	6.81		6.32
Exercisable at September 30, 2018		4,189		$	6.42		4.99
Vested and expected to vest at September 30, 2018		5,942		$	6.76		6.10

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

	Three Months Ended						Nine Months Ended
	September 30,						September 30,
	2018			2017			2018			2017
Risk-free interest rate		2.81	%		1.82	%		2.71	%		2.17	%
Expected life (in years)		5.23			5.19			6.68			6.76
Expected volatility		62	%		74	%		69	%		72	%
Expected dividend yield		--			--			--			--

Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2018		2017		2018		2017
Research and development expenses	$	515	$	380	$	1,492	$	941
Selling, general and administrative expenses		608		555		2,809		2,183
Total stock-based compensation expense	$	1,123	$	935	$	4,301	$	3,124