Collaboration arrangements and concentration of credit risk - BMS collaboration (Details) $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
	May 21, 2015 USD ($)	Aug. 31, 2015 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)
Collaboration arrangements
Revenue			$ 3,148	$ 2,260	$ 9,676	$ 10,523
Minimum
Collaboration arrangements
Decrease in revenue					1,800
Maximum
Collaboration arrangements
Decrease in revenue					5,300
BMS
Collaboration arrangements
Revenue			3,148	$ 2,260	$ 9,676	$ 5,877
License revenue | BMS
Collaboration arrangements
Number of potential targets included in collaborative agreement | item					10
Upfront payment recorded	$ 60,100
Royalty term after the first commercial sale					10 years
Average period for completion of pre-clinical phase					4 years
Average period for completion of clinical development and commercial launch					8 years 6 months
Percentage of increase in probability of all additional targets					10
License revenue | BMS | First Target Selection
Collaboration arrangements
Milestone payments to be received upon achievement					$ 254,000
License revenue | BMS | Second, Third, and Fourth Targets Selection
Collaboration arrangements
Target designation payment received		$ 15,000
License revenue | BMS | Fifth through Tenth Targets Selection
Collaboration arrangements
Maximum target designation payments to which entitled per agreement					16,500
License revenue | BMS | Other Selected Targets
Collaboration arrangements
Milestone payments to be received upon achievement					217,000
Before Topic 606
Collaboration arrangements
Revenue			$ 1,000		$ 3,200