Warrant liability - Roll-forward of fair values of warrant liability (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Roll-forward of the fair values of the Company’s warrant liability
Fair value at the beginning	$ 4,804	$ 4,804
Change in fair value of warrant liability	5,381	5,381	$ 665	$ 119
Conversion of warrant liability to equity upon closing of IPO and exercise of warrants	(10,021)	$ (10,027)
Impact of exchange rates on translation of warrant liability to USD included in accumulated other comprehensive income (loss)	$ (164)
Fair value at the end			$ 4,804