Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Options disclosures
Stock-based compensation	$ 3,349	$ 2,636	$ 12,120	$ 13,072
Stock options granted (in shares)			1,538,515
Unvested stock options
Options disclosures
Unrecognized stock-based compensation expense	29,300		$ 29,300
Remaining weighted-average remaining vesting period			2 years 8 months 12 days
Stock options
Options disclosures
Stock-based compensation	$ 3,008	$ 2,559	$ 10,312	$ 12,428
Share-based Payment Arrangement, Employee
Fair value assumptions
Risk-free interest rate	1.60%	2.80%	2.30%	2.70%
Expected term	6 years	6 years	6 years	6 years
Expected volatility	74.10%	74.50%	74.90%	74.40%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 13.17	$ 12.20	$ 10.29	$ 14.37
Stock-based compensation	$ 3,000	$ 2,500	$ 10,400	$ 12,300
Share-based Payment Arrangement, Nonemployee
Options disclosures
Stock options granted (in shares)	0		0
Share-based Payment Arrangement, Nonemployee | Stock options
Options disclosures
Stock-based compensation	$ 100	$ 100	$ 100	$ 300