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Note 7 - Stock-based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
Notes Tables  
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]
    Year Ended December 31,
    2017   2016   2015
Research and development   $
9,602
     $
6,088
     $
7,580
  
General and administrative    
3,019
      
2,399
      
2,125
  
Total stock-based compensation expense   $
12,621
     $
8,487
     $
9,705
  
Share-based Compensation, Activity [Table Text Block]
    Awards
Available		   Options
Outstanding		   Weighted
Average
Exercise
Price
Balance at December 31, 2014    
2,362
      
9,605
     $
6.21
  
Restricted stock awards granted    
(163
 )    
—
      
—
  
Restricted stock awards cancelled    
1
      
—
      
—
  
Stock option awards granted    
(2,217
 )    
2,217
      
11.52
  
Stock option awards exercised    
—
      
(1,118
 )    
4.36
  
Stock option awards cancelled    
33
      
(33
 )    
9.87
  
Balance at December 31, 2015    
16
      
10,671
      
7.50
  
Plan amendment    
3,800
      
—
      
—
  
Restricted stock awards granted    
(34
 )    
—
      
—
  
Restricted stock awards cancelled    
22
      
—
      
—
  
Stock option awards granted    
(2,248
 )    
2,248
      
3.20
  
Stock option awards exercised    
—
      
(107
 )    
2.63
  
Stock option awards cancelled    
717
      
(717
 )    
10.78
  
Balance at December 31, 2016    
2,273
      
12,095
      
6.55
  
Plan amendment    
1,000
      
—
      
—
  
Restricted stock awards granted    
(22
 )    
—
      
—
  
Restricted stock awards cancelled    
12
      
—
      
—
  
Stock option awards granted    
(3,915
 )    
3,915
      
5.33
  
Stock option awards exercised    
—
      
(438
 )    
3.50
  
Stock option awards cancelled    
1,120
      
(1,120
 )    
9.72
  
Balance at December 31, 2017    
468
      
14,452
     $
6.06
  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
    2017   2016   2015
Expected Life    
5.5
      
5.5
      
5.5
  
Expected Volatility    
82
 %    
82
 %    
81
 %
Expected Dividend Yield    
0.0
 %    
0.0
 %    
0.0
 %
Risk-Free Interest Rate    
2.0
 %    
1.4
 %    
1.6
 %
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding and Exercisable [Table Text Block]
          Outstanding   Exercisable
Range       Number   Weighted
Average
Remaining
Life		   Weighted
Average
Exercise
Price		   Number   Weighted
Average
Exercise
Price
$ 0
to
3
  
1,724
    
4.7
     $
1.64
      
1,598
     $
1.55
  
  3
to
6
  
8,303
    
7.5
      
4.63
      
3,066
      
4.31
  
  6
to
9
  
854
    
3.0
      
6.90
      
848
      
6.89
  
  9
to
12
  
2,857
    
7.1
      
11.00
      
1,383
      
10.98
  
  12
to
15
  
619
    
6.9
      
12.29
      
308
      
12.39
  
  15
to
18
  
95
    
7.5
      
15.39
      
47
      
15.39
  
$ 0
to
18
  
14,452
    
6.8
     $
6.06
      
7,250
     $
5.69
  
Share-based Compensation, Stock Options, Activity [Table Text Block]
    Non-Vested
Stock Option
Awards		   Weighted Average
Grant-Date Fair
Value
Balance December 31, 2016    
5,598
     $
5.40
  
Stock option awards granted    
3,915
      
3.63
  
Stock option awards vested    
(1,916
 )    
4.86
  
Stock option awards forfeited    
(395
 )    
5.84
  
Balance December 31, 2017    
7,202
     $
4.56