Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair value assumptions
Expected dividend yield	0.00%
Options disclosures
Stock-based compensation	$ 15,710	$ 9,238	$ 6,310
Stock options granted (in shares)	2,215,891
Unrecognized stock-based compensation expense	$ 26,100
Remaining weighted-average remaining vesting period	2 years 9 months 11 days
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	2.80%	2.00%	1.50%
Expected term	6 years	6 years	6 years
Expected volatility	74.40%	73.70%	73.10%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 13.87
Stock-based compensation	$ 14,900	$ 5,500	$ 3,000
Non-employees stock options
Fair value assumptions
Risk-free interest rate	2.60%	2.40%	2.10%
Expected term	7 years 7 months 6 days	8 years 6 months	9 years 1 month 6 days
Expected volatility	73.20%	76.20%	83.30%
Options disclosures
Stock-based compensation	$ 300	$ 400	$ 200
Stock options granted (in shares)	0