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Capital structure (Details 2)
$ / shares in Units, $ in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended
Oct. 16, 2014
USD ($)
$ / shares
shares
Mar. 07, 2013
USD ($)
shares
Feb. 28, 2014
USD ($)
$ / shares
shares
Jun. 30, 2013
USD ($)
$ / shares
shares
May. 31, 2013
USD ($)
$ / shares
shares
Mar. 31, 2013
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2015
shares
Dec. 31, 2014
shares
Feb. 28, 2013
USD ($)
$ / shares
shares
Capital structure                    
Gross proceeds raised | $           $ 60,000        
Gain on extinguishment of shares of Series One, Two and Three stock | $             $ 3,391      
Aggregate purchase price | $             $ 131,650      
Common stock                    
Common stock, authorized shares               125,000,000 125,000,000  
Series One and Series Two | Warrants                    
Capital structure                    
Warrants issued to purchase shares                   2,527,675
Exercise price (in dollars per share) | $ / shares                   $ 0.01
Shares issued and sold   2,527,675                
Series Four convertible preferred stock                    
Capital structure                    
Shares issued and sold   4,497,035     375,000          
Aggregate gross proceeds | $   $ 54,000                
Share price (in dollars per share) | $ / shares         $ 12.00          
Aggregate purchase price | $         $ 4,500          
Series Five convertible preferred stock                    
Capital structure                    
Number of shares issued upon conversion   6,700,487                
Common stock                    
Number of shares issued upon conversion   6,700,487                
Series Five convertible preferred stock | Warrants                    
Capital structure                    
Number of shares issued upon conversion   2,095,515                
Common stock                    
Number of shares issued upon conversion   2,095,515                
Common stock                    
Capital structure                    
Shares issued and sold 3,450,000   5,163,265 9,627,800            
Reverse stock split ratio   0.0083                
Public offering price (in dollars per share) | $ / shares $ 36.25   $ 24.50 $ 15.00            
Common stock                    
Common stock, authorized shares         125,000,000          
Common stock shares issuable pursuant to the exercise by the underwriters of an over-allotment option 450,000   673,469 1,255,800            
Net proceeds from public offerings | $ $ 117,600   $ 118,400              
Net proceeds from initial public offering | $       $ 131,600            
Series Four and Series Five Convertible Preferred Stock                    
Capital structure                    
Number of shares issuable upon conversion of each stock       1            
Number of shares issued upon conversion       14,170,956            
Common stock                    
Number of shares issued upon conversion       14,170,956            
Convertible promissory notes                    
Capital structure                    
Principal amount of Notes | $                   $ 6,000
Debt discount recorded to interest expense | $           6,000        
Amount raised in bridge financing | $           $ 6,000        
Convertible promissory notes | Series Four convertible preferred stock                    
Capital structure                    
Shares issued upon conversion of convertible promissory notes   502,919