Annual report pursuant to Section 13 and 15(d)

Stockholder's Equity - Options Outstanding (Details)

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Stockholder's Equity - Options Outstanding (Details) - $ / shares
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Number of Shares      
Balance at the end of the period (in shares) 4,630,073    
Options outstanding      
Number of Shares      
Balance at the beginning of the period (in shares) 3,574,450 2,795,746  
Granted (in shares) 1,645,960 1,095,042  
Exercised (in shares) (384,033) (186,687)  
Forfeitures (in shares) (206,304) (129,651)  
Balance at the end of the period (in shares) 4,630,073 3,574,450 2,795,746
Vested or expected to vest at the end of the period (in shares) 4,488,894    
Exercisable at the end of the period (in shares) 1,941,347    
Weighted-Average Exercise Price      
Balance at the beginning of the period (in dollars per share) $ 3.75 $ 2.52  
Granted (in dollars per share) 7.16 6.75  
Exercised (in dollars per share) 2.36 0.60  
Forfeitures/Expirations (in dollars per share) 6.04 7.20  
Balance at the end of the period (in dollars per share) 4.98 $ 3.75 $ 2.52
Vested or expected to vest at the end of the period (in dollars per share) 4.91    
Exercisable at the end of the period (in dollars per share) $ 3.10    
Weighted Average Remaining Contractual Term      
Options Outstanding at the end of the period 7 years 10 months 13 days 8 years 22 days 8 years 5 months 12 days