Collaboration arrangements and concentration of credit risk - Amounts owed by BMS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Number of ordinary shares held	44,444,405		44,444,405		43,711,954
Amounts owed in relation to the collaboration services	$ 222		$ 222		$ 947
Total revenues	1,535	$ 2,474	1,639	$ 3,610
BMS arrangement
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues			222		947
BMS arrangement | Bristol Myers Squibb
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Amounts owed in relation to the collaboration services	$ 222		$ 222		$ 947