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Note 6 - Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2015
Notes Tables  
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]
    Year Ended December 31,
    2015   2014   2013
Research and development   $ 7,580     $ 8,906     $ 3,664  
General and administrative     2,125       1,271       704  
Total stock-based compensation expense   $ 9,705     $ 10,177     $ 4,368  
Schedule of Share-based Compensation, Activity [Table Text Block]
    Awards
Available		   Options
Outstanding		   Weighted
Average
Exercise
Price
Balance at December 31, 2012     2,815       8,073     $ 6.09  
Restricted stock awards granted     (310 )     —       —  
Restricted stock awards cancelled     53       —       —  
Stock option awards granted     (3,277 )     3,277       3.05  
Stock option awards exercised     —       (563 )     2.37  
Stock option awards cancelled     1,801       (1,801 )     7.22  
Balance at December 31, 2013     1,082       8,986       4.99  
Plan amendment     3,750       —       —  
Restricted stock awards granted     (593 )     —       —  
Restricted stock awards cancelled     —       —       —  
Stock option awards granted     (1,965 )     1,965       10.99  
Stock option awards exercised     —       (1,258 )     4.78  
Stock option awards cancelled     88       (88 )     8.83  
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  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
    2015   2014   2013
Expected Life     5.5       5.5       4.7  
Expected Volatility     81 %     87 %     84 %
Expected Dividend Yield     0.0 %     0.0 %     0.0 %
Risk-Free Interest Rate     1.6 %     1.6 %     0.7 %
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
 
1,826
 
 
6.4
 
 
$
1.51
 
 
 
1,322
 
 
$
1.55
 
 
3
to
6
 
2,792
 
 
6.1
 
 
 
4.63
 
 
 
2,329
 
 
 
4.53
 
 
6
to
9
 
1,153
 
 
4.5
 
 
 
7.10
 
 
 
1,070
 
 
 
7.13
 
 
9
to
12
 
3,728
 
 
8.1
 
 
 
11.07
 
 
 
714
 
 
 
11.45
 
 
12
to
15
 
1,077
 
 
6.4
 
 
 
12.44
 
 
 
374
 
 
 
12.65
 
 
15
to
18
 
95
 
 
9.5
 
 
 
15.39
 
 
 
—
 
 
 
—
 
$
0
to
18
 
10,671
 
 
6.7
 
 
$
7.50
 
 
 
5,809
 
 
$
5.70
 
Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block]
    Non-Vested
Stock Option
Awards		   Weighted Average
Grant-Date Fair
Value
Balance December 31, 2014     4,385     $ 4.27  
Stock option awards granted     2,217       7.72  
Stock option awards vested     (1,711 )     2.63  
Stock option awards forfeited     (28 )     6.39  
Balance December 31, 2015     4,863     $ 6.40