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Stock-based compensation - Stock options (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Additional disclosures          
Stock-based compensation expense, reversed $ 1,300   $ 2,100    
Stock options          
Shares          
Outstanding at the beginning of the period (in shares)     4,650,114    
Granted (in shares) 446,450 1,006,100 1,487,100 1,425,600  
Exercised (in shares)     (24,663)    
Forfeited/Canceled (in shares)     (844,303)    
Outstanding at the end of the period (in shares) 5,268,248   5,268,248   4,650,114
Exercisable at the end of the period (in shares) 2,052,413   2,052,413    
Vested and expected to vest at the end of the period (in shares) 4,542,808   4,542,808    
Weighted average exercise price per share          
Outstanding at the beginning of the period (in dollars per share)     $ 21.49    
Granted (in dollars per share) $ 7.32 $ 36.19 7.48 $ 37.93  
Exercised (in dollars per share)     4.52    
Forfeited/Canceled (in dollars per share)     32.89    
Outstanding at the end of period (in dollars per share) 15.79   15.79   $ 21.49
Exercisable at the end of the period (in dollars per share) 17.49   17.49    
Vested and expected to vest at the end of the period (in dollars per share) $ 15.96   $ 15.96    
Weighted Average Remaining contractual Term (years)          
Outstanding at the beginning of the period     8 years 5 months 10 days   8 years 6 months 29 days
Outstanding at the end of the period     8 years 5 months 10 days   8 years 6 months 29 days
Exercisable at the end of the period     7 years 7 months 15 days    
Vested and expected to vest at the end of the period     8 years 4 months 12 days    
Aggregate intrinsic value          
Outstanding at the beginning of the period     $ 2,970    
Outstanding at the end of period $ 278   278   $ 2,970
Exercisable at the end of period 278   278    
Vested and expected to vest at the end of the period $ 278   $ 278    
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model          
Dividend yield 0.00% 0.00% 0.00% 0.00%  
Volatility   73.00%      
Additional disclosures          
Total intrinsic value of options exercised $ 100 $ 1,600 $ 100 $ 6,500  
Total unrecognized compensation cost, net of estimated forfeitures, related to unvested stock options and restricted stock units $ 19,600   $ 19,600    
Weighted average period for recognition of compensation cost related to unvested stock awards     2 years 239 days    
Weighted average grant date fair value (in dollars per share) $ 5.28 $ 23.73 $ 5.27 $ 24.00  
Stock options | Minimum          
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model          
Risk-free interest rate 1.40% 1.60% 1.40% 1.60%  
Volatility 86.00%   78.00% 73.00%  
Expected term (years) 5 years 3 months 18 days 5 years 3 months 18 days 5 years 3 months 18 days 5 years 3 months 18 days  
Stock options | Maximum          
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model          
Risk-free interest rate 1.50% 2.30% 2.00% 2.30%  
Volatility 89.00%   89.00% 75.00%  
Expected term (years) 6 years 1 month 6 days 9 years 10 months 24 days 9 years 10 months 24 days 9 years 10 months 24 days