Note 11 - Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 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		1,566		$	6.93
Exercised		(96	)	$	4.85
Cancelled		(483	)	$	7.97
Expired		(258	)	$	16.05
Outstanding at December 31, 2018		6,264		$	6.79		6.02
Exercisable at December 31, 2018		4,242		$	6.42		4.78
Vested and expected to vest at December 31, 2018		5,887		$	6.75		5.84

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

	2018		2017		2016
Risk-free interest rate		2.72%		2.17%		1.53%
Expected life (in years)		6.67		6.76		6.77
Expected volatility		69%		72%		74%
Expected dividend yield		--		--		--

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

	2018		2017		2016
Research and development expenses	$	1,760	$	1,371	$	843
General and administrative expenses		3,449		2,771		1,614
Total stock-based compensation expense	$	5,209	$	4,142	$	2,457