Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Options disclosures
Stock-based compensation	$ 2,322	$ 2,165
Stock options granted (in shares)	926,091
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	2.70%	2.00%
Expected term	6 years	6 years
Expected volatility	74.30%	73.60%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 16.49	$ 7.82
Stock-based compensation	$ 1,900	$ 1,300
Non-employees stock options
Fair value assumptions
Risk-free interest rate	2.70%	2.00%
Expected term	8 years 4 months 24 days	8 years 8 months 12 days
Expected volatility	76.10%	83.60%
Options disclosures
Stock-based compensation	$ 100	$ 200
Stock options granted (in shares)	0	0
Unvested stock options
Options disclosures
Unrecognized stock-based compensation expense	$ 28,200
Remaining weighted-average remaining vesting period	2 years 9 months 18 days