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Stock award plan - Share Base Compensation (Details) - Stock option
$ / shares in Units, $ in Thousands
 3 Months Ended
Mar. 31, 2017
USD ($)
$ / shares
shares
Number of options  
Outstanding at the beginning of the period (in shares) | shares 5,854,316
Granted (in shares) | shares 1,244,723
Exercised (in shares) | shares (9,766)
Forfeited/Cancelled (in shares) | shares (81,983)
Outstanding at the end of the period (in shares) | shares 7,007,290
Vested or Expected to vest at the end of the period (in shares) | shares 3,296,281
Exercisable at the end of the period (in shares) | shares 3,478,720
Weighted- average exercise price  
Outstanding at the beginning of the period (in dollars per share) | $ / shares $ 34.71
Granted (in dollars per share) | $ / shares 11.32
Exercised (in dollars per share) | $ / shares 10.85
Forfeited/Cancelled (in dollars per share) | $ / shares 35.64
Outstanding at the end of the period (in dollars per share) | $ / shares 30.56
Vested or Expected to vest at the end of the period (in dollars per share) | $ / shares 28.16
Exercisable at the end of the period (in dollars per share) | $ / shares $ 33.28
Weighted- average remaining contractual term  
Outstanding at the end of the period 7 years 10 months 13 days
Vested or Expected to vest at the end of the period 8 years 8 months 4 days
Exercisable at the end of the period 7 years 11 days
Aggregate intrinsic value  
Outstanding at the end of the period (in dollars) | $ $ 179
Vested or Expected to vest at the end of the period (in dollars) | $ 144
Exercisable at the end of the period (in dollars) | $ $ 25
Minimum  
Valuation assumptions  
Risk-free interest rate (as a percent) 1.94%
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