Collaboration arrangements and concentration of credit risk - Amounts owed by BMS (Details) $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	May 31, 2015 item	Mar. 31, 2019 USD ($) item shares	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) shares
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Common Stock, Shares, Outstanding | shares		37,763,842		37,351,653
Total		$ 1,136	$ 3,478
Amounts owed in relation to the collaboration services		461		$ 233
BMS arrangement
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total		461		233
BMS arrangement | Bristol Myers Squibb
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Number of collaboration targets | item	4
Amounts owed in relation to the collaboration services		$ 461		$ 233
BMS arrangement | Maximum | Bristol Myers Squibb
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Number of collaboration targets | item		10