Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Options disclosures
Stock-based compensation	$ 2,636	$ 3,241	$ 13,072	$ 6,945
Stock options granted (in shares)			2,030,191
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	2.80%	1.80%	2.70%	2.00%
Expected term	6 years	6 years	6 years	6 years
Expected volatility	74.50%	72.90%	74.40%	73.80%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 12.20	$ 9.43	$ 14.37	$ 7.88
Stock-based compensation	$ 2,500	$ 1,400	$ 12,300	$ 3,800
Non-employees stock options
Fair value assumptions
Risk-free interest rate			3.00%	2.20%
Expected term			7 years 10 months 24 days	8 years 8 months 12 days
Expected volatility			73.90%	77.50%
Options disclosures
Stock-based compensation	$ 100	$ 100	$ 300	$ 300
Stock options granted (in shares)	0	20,000	0	20,000
Unvested stock options
Options disclosures
Unrecognized stock-based compensation expense	$ 27,700		$ 27,700
Remaining weighted-average remaining vesting period			2 years 10 months 24 days