Collaboration arrangements and concentration of credit risk - Amounts owed by BMS (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Number of ordinary shares held	44,299,596		43,711,954
Amounts owed in relation to the collaboration services	$ 258		$ 947
Total revenues	104	$ 1,136
BMS arrangement
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenues	258		947
BMS arrangement | Bristol Myers Squibb
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Amounts owed in relation to the collaboration services	$ 258		$ 947