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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2014
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]  
Summary of Option Activity

The following is a summary of option activity under the 2005 Plan and the 1995 Plan for the six months ended September 30, 2014:

 

  2005 Stock Incentive Plan     1995 Stock Option Plan  
  Stock
Options		     Weighted-
Average
Exercise
Price		     Stock
Options		     Weighted-
Average
Exercise
Price		  
Outstanding at January 1, 2014     11,788,100     $ 1.87       188,150     $ 5.04  
Granted     5,985,500     $ 5.48       —     $ —  
Exercised     (524,912 )   $ 1.90       (32,450 )   $ 4.81  
Canceled     (268,775 )   $ 3.17       (149,100 )   $ 5.76  
Outstanding at September 30, 2014     16,979,913     $ 3.12       6,600     $ 2.21  
Shares exercisable at September 30, 2014     6,265,762     $ 2.01       6,600     $ 2.21  
                                 
Shares available for grant at September 30, 2014     4,896,344                        
Assumptions used to Estimate Grant Date Fair Value of Stock Options granted using Black-Scholes Option-Pricing Model

The fair value of stock options granted was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:

 

    Three Months Ended
September 30,		     Nine Months Ended
 September 30,
    2014     2013     2014     2013
Weighted-average Black-Scholes fair value of stock options granted     $ 1.92       $ 1.41       $ 2.41       $ 1.02  
Risk-free interest rate       1.43%-1.51%         1.12%-1.36%         1.24%-2.22%         0.54%-1.36%  
Dividend yield       0%       0%       0%       0%
Volatility       52.75%-54.25%         56.75%-62.51%         52.47%-67.93%         55.81%-73.72%  
Expected term (in years)       4.10-4.27         4.25         4.04-6.96         3.98-7.05  
Expected forfeiture rate       14.18%-16.33%         23.15%         0%-23.15%         0%-23.15%  
Stock-Based Compensation Expense

The Company recorded stock-based compensation expense in the consolidated statements of operations as follows (in thousands):

 

 

Three Months Ended
September 30,

    

Nine Months Ended

September 30,

  
  2014     2013     2014     2013  
Research and development   $ 780     $ 316     $ 1,955     $ 829  
General and administrative     911       347       2,628       955  
Total stock-based
    compensation expense		   $ 1,691     $ 663     $ 4,583     $ 1,784  
Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the nine months ended September 30, 2014:

 

 

Number of

Shares

     Per Share
Weighted-
Average
Grant-Date
Fair Value		  
Outstanding and Unvested at January 1, 2014     16,667     $ 1.39  
Restricted stock granted     15,000     $ 4.48  
Restricted stock vested     —     $ —  
Restricted stock forfeited     —     $ —  
Outstanding and Unvested at September 30, 2014     31,667     $ 2.85  

 
Employee Stock Purchase Plan [Member]
  
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]  
Assumptions used to Estimate Grant Date Fair Value of Stock Options granted using Black-Scholes Option-Pricing Model

The ESPP is considered compensatory for financial reporting purposes. As such, the fair value of ESPP shares was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:

 

   

Three Months

Ended

September 30,

2014

    

Nine Months

Ended

September 30,

2014
Range of Black-Scholes fair values of ESPP shares granted       $0.97-$2.08         $0.78-$2.08  
Risk-free interest rate       0.05%-0.24%         0.04%-0.24%  
Dividend yield       0%       0%
Volatility       51.10%-67.57%         50.80%-67.57%  
Expected term (in years)       0.5-1.5         0.5-1.5  
Expected forfeiture rate       5%       5%