Stock award plan - Share Base Compensation (Details) - Stock option $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($) $ / shares shares
Number of options
Outstanding at the beginning of the period (in shares) | shares	4,826,477
Granted (in shares) | shares	1,440,445
Exercised (in shares) | shares	(85,325)
Forfeited/Cancelled (in shares) | shares	(349,431)
Outstanding at the end of the period (in shares) | shares	5,832,166
Vested or Expected to vest at the end of the period (in shares) | shares	3,032,483
Exercisable at the end of the period (in shares) | shares	2,613,982
Weighted-average exercise price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 37.20
Granted (in dollars per share) | $ / shares	28.55
Exercised (in dollars per share) | $ / shares	10.85
Forfeited/Cancelled (in dollars per share) | $ / shares	46.77
Outstanding at the end of the period (in dollars per share) | $ / shares	35.03
Vested or Expected to vest at the end of the period (in dollars per share) | $ / shares	35.79
Exercisable at the end of the period (in dollars per share) | $ / shares	$ 34.08
Weighted-average remaining contractual term
Outstanding at the end of the period	8 years 4 days
Vested or Expected to vest at the end of the period	8 years 6 months
Exercisable at the end of the period	7 years 4 months 17 days
Aggregate intrinsic value
Outstanding at the end of the period (in dollars) | $	$ 4,132
Vested or Expected to vest at the end of the period (in dollars) | $	1,200
Exercisable at the end of the period (in dollars) | $	$ 2,867
Minimum
Valuation assumptions
Risk-free interest rate (as a percent)	1.30%
Expected volatility (as a percent)	67.00%
Expected term	5 years 18 days
Maximum
Valuation assumptions
Risk-free interest rate (as a percent)	2.24%
Expected volatility (as a percent)	78.00%
Expected term	10 years