Note 2 - Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2016

Notes Tables

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

	Year Ended December 31,
	2016		2015		2014
Research and development	$	2,095	$	2,204	$	1,303
General and administrative		3,478		2,203		5,717
Total share-based compensation expense	$	5,573	$	4,407	$	7,020

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

	Year Ended December  31,
	2016			2015			2014
Weighted-average grant date fair value per share		$7.29			$17.95			$8.23
Expected volatility	65%	-	70%	66%	-	71%	70%	-	73%
Risk-free interest rate	1.09%	-	2.44%	1.07%	-	2.42%	0.69%	-	2.73%
Expected life (in years)	5	-	10	3	-	10	3	-	10
Dividend yield		—			—			—