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Stock award plan - Share Base Compensation (Details) - Stock option
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2019
USD ($)
$ / shares
shares
Number of options  
Outstanding at the beginning of the period (in shares) | shares 8,534,358
Granted (in shares) | shares 2,490,970
Exercised (in shares) | shares (80,826)
Forfeited/Cancelled (in shares) | shares (133,119)
Outstanding at the end of the period (in shares) | shares 10,811,383
Vested or Expected to vest at the end of the period (in shares) | shares 5,364,300
Exercisable at the end of the period (in shares) | shares 4,987,460
Weighted- average exercise price  
Outstanding at the beginning of the period (in dollars per share) | $ / shares $ 28.58
Granted (in dollars per share) | $ / shares 33.01
Exercised (in dollars per share) | $ / shares 15.85
Forfeited/Cancelled (in dollars per share) | $ / shares 29.10
Outstanding at the end of the period (in dollars per share) | $ / shares 29.67
Vested or Expected to vest at the end of the period (in dollars per share) | $ / shares 28.40
Exercisable at the end of the period (in dollars per share) | $ / shares $ 31.00
Weighted- average remaining contractual term  
Outstanding at the end of the period 7 years 8 months 19 days
Vested or Expected to vest at the end of the period 9 years 1 month 24 days
Exercisable at the end of the period 6 years 15 days
Aggregate intrinsic value  
Outstanding at the end of the period (in dollars) | $ $ 117,201
Vested or Expected to vest at the end of the period (in dollars) | $ 53,343
Exercisable at the end of the period (in dollars) | $ $ 60,063
Minimum  
Valuation assumptions  
Risk-free interest rate (as a percent) 2.53%
Expected volatility (as a percent) 63.00%
Expected term 6 years 1 month 9 days
Maximum  
Valuation assumptions  
Risk-free interest rate (as a percent) 2.63%
Expected volatility (as a percent) 63.00%
Expected term 6 years 1 month 9 days