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Stock award plan - Share Base Compensation (Details) - Stock option
$ / shares in Units, $ in Thousands
 6 Months Ended
Jun. 30, 2017
USD ($)
$ / shares
shares
Number of options  
Outstanding at the beginning of the period (in shares) | shares 5,854,316
Granted (in shares) | shares 1,738,873
Exercised (in shares) | shares (49,770)
Forfeited/Cancelled (in shares) | shares (419,367)
Outstanding at the end of the period (in shares) | shares 7,124,052
Vested or Expected to vest at the end of the period (in shares) | shares 3,159,956
Exercisable at the end of the period (in shares) | shares 3,738,673
Weighted- average exercise price  
Outstanding at the beginning of the period (in dollars per share) | $ / shares $ 34.71
Granted (in dollars per share) | $ / shares 11.80
Exercised (in dollars per share) | $ / shares 10.74
Forfeited/Cancelled (in dollars per share) | $ / shares 33.83
Outstanding at the end of the period (in dollars per share) | $ / shares 29.33
Vested or Expected to vest at the end of the period (in dollars per share) | $ / shares 25.63
Exercisable at the end of the period (in dollars per share) | $ / shares $ 32.94
Weighted- average remaining contractual term  
Outstanding at the end of the period 7 years 6 months 2 days
Vested or Expected to vest at the end of the period 8 years 7 months 27 days
Exercisable at the end of the period 6 years 5 months 10 days
Aggregate intrinsic value  
Outstanding at the end of the period (in dollars) | $ $ 19,807
Vested or Expected to vest at the end of the period (in dollars) | $ 10,427
Exercisable at the end of the period (in dollars) | $ $ 8,449
Minimum  
Valuation assumptions  
Risk-free interest rate (as a percent) 1.84%
Expected volatility (as a percent) 76.00%
Expected term 5 years 16 days
Maximum  
Valuation assumptions  
Risk-free interest rate (as a percent) 2.45%
Expected volatility (as a percent) 81.00%
Expected term 10 years