Stock award plan - Share Base Compensation (Details) - Stock option $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 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	1,809,873
Exercised (in shares) | shares	(132,795)
Forfeited/Cancelled (in shares) | shares	(918,629)
Outstanding at the end of the period (in shares) | shares	6,612,765
Vested or Expected to vest at the end of the period (in shares) | shares	2,719,750
Exercisable at the end of the period (in shares) | shares	3,718,713
Weighted- average exercise price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 34.71
Granted (in dollars per share) | $ / shares	12.12
Exercised (in dollars per share) | $ / shares	10.82
Forfeited/Cancelled (in dollars per share) | $ / shares	33.20
Outstanding at the end of the period (in dollars per share) | $ / shares	29.21
Vested or Expected to vest at the end of the period (in dollars per share) | $ / shares	24.50
Exercisable at the end of the period (in dollars per share) | $ / shares	$ 33.07
Weighted- average remaining contractual term
Outstanding at the end of the period	7 years 5 months 5 days
Vested or Expected to vest at the end of the period	8 years 7 months 2 days
Exercisable at the end of the period	6 years 6 months 3 days
Aggregate intrinsic value
Outstanding at the end of the period (in dollars) | $	$ 22,786
Vested or Expected to vest at the end of the period (in dollars) | $	11,858
Exercisable at the end of the period (in dollars) | $	$ 10,014
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