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Compensation and Other Benefit Plans (Tables)
 12 Months Ended
Dec. 31, 2014
Compensation and Other Benefit Plans [Abstract]  
Stock option activity
      The following table summarizes the Company’s stock option activity:
 
  
2014
  
2013
  
2012
 
  
Number of
Shares
  
Weighted
Average
Exercise
Price
  
Number of
Shares
  
Weighted
Average
Exercise
Price
  
Number of
Shares
  
Weighted
Average
Exercise
Price
 
Outstanding at beginning of year
  
7,216,041
  
$
8.42
   
6,788,383
  
$
8.99
   
5,053,435
  
$
12.55
 
Granted
  
1,891,989
   
6.69
   
1,168,203
   
3.13
   
2,351,445
   
2.59
 
Exercised
  
(915,911
)
  
3.91
   
(589,355
)
  
2.26
   
(90,252
)
  
1.68
 
Forfeited, expired or cancelled
  
(489,810
)
  
14.36
   
(151,190
)
  
17.46
   
(526,245
)
  
15.84
 
Outstanding at end of year
  
7,702,309
   
8.15
   
7,216,041
   
8.42
   
6,788,383
   
8.99
 
Exercisable at end of year
  
4,908,925
   
9.98
   
4,814,926
   
11.14
   
4,276,834
   
12.42
 
Weighted average grant date fair value
     
$
4.49
      
$
2.27
      
$
1.89
 
Unvested RSU activity
Unvested RSU activity for the year ended December 31, 2014 is summarized below:

    
Weighted
 
  
Number of
  
Average Grant
 
  
Shares
  
Date Fair Value
 
     
Unvested awards at December 31, 2013
  
1,738,037
  
$
2.73
 
Granted
  
1,506,194
   
7.03
 
Vested
  
(1,099,701
)
  
3.51
 
Forfeited
  
(190,651
)
  
4.22
 
Unvested awards at December 31, 2014
  
1,953,879
   
5.46
 
Weighted average assumptions
The fair value of stock option awards was estimated using the Black-Scholes model with the following weighted average assumptions for the years ended December 31, 2014, 2013, and 2012:

  
Year Ended December 31,
 
  
2014
  
2013
  
2012
 
Dividend yield
  
0
%
  
0
%
  
0
%
Expected volatility
  
92
%
  
92
%
  
92
%
Risk-free interest rate
  
1.72
%
  
0.89
%
  
0.82
%
Expected term
 
5.6 years
  
5.6 years
  
5.6 years
 
Stock-based compensation expense
The following table shows total stock-based compensation expense included in the accompanying consolidated statements of comprehensive loss for the years ended December 31, 2014, 2013, and 2012 (in thousands):

  
Year Ended December 31,
 
  
2014
  
2013
  
2012
 
Research and development
 
$
5,557
  
$
2,358
  
$
2,391
 
Selling, general and administrative
  
5,215
   
2,741
   
1,893
 
Total stock-based compensation expense
 
$
10,772
  
$
5,099
  
$
4,284