Warrant liability - Roll-forward of fair values of warrant liability (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Jun. 30, 2019	Sep. 30, 2019	Sep. 30, 2018
Roll-forward of the fair values of the Company’s warrant liability
Fair value at the beginning		$ 4,804
Change in fair value of warrant liability recorded as other expense		5,381	$ 1,472
Conversion of warrant liability to equity upon closing of IPO and exercise of warrants	$ (10,027)	(10,021)
Impact of exchange rates on translation of warrant liability to USD included in accumulated other comprehensive income (loss)		$ (164)