Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Options disclosures
Stock-based compensation	$ 4,040	$ 3,518
Stock options granted (in shares)	899,085
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	1.40%	2.50%
Expected term	6 years	6 years
Expected volatility	72.30%	75.00%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 8.25	$ 6.23
Stock-based compensation	$ 3,200	$ 2,800
Non-employees stock options
Options disclosures
Stock-based compensation	$ 100	$ 100
Stock options granted (in shares)	0	0
Unvested stock options
Options disclosures
Unrecognized stock-based compensation expense	$ 30,800
Remaining weighted-average remaining vesting period	2 years 9 months 18 days
Outstanding stock options
Options disclosures
Stock-based compensation	$ 3,208	$ 2,776