Collaboration arrangements and concentration of credit risk - BMS collaboration - Narrative (Details) $ in Thousands			1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 24, 2020 USD ($)	May 21, 2015 USD ($)	Feb. 28, 2019	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) item	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Collaboration arrangements
Revenue				$ 1,535	$ 2,474	$ 1,639	$ 3,610
CSL Behring collaboration
Collaboration arrangements
Upfront cash to receive upon the closing of the agreement	$ 450,000					450,000
Regulatory and commercial milestone payments received	$ 1,600,000
CSL Behring collaboration | Transaction Payment Received in 2020
Collaboration arrangements
Upfront cash to receive upon the closing of the agreement						$ 450,000
Tax rate enacted in the Netherlands						25.00%
CSL Behring collaboration | Transaction payment received thereafter
Collaboration arrangements
Upfront cash to receive upon the closing of the agreement						$ 450,000
Tax rate enacted in the Netherlands						21.70%
License revenues from related party
Collaboration arrangements
Revenue				1,530	2,108	$ 1,577	2,665
Collaboration revenues from related party
Collaboration arrangements
Revenue				$ 5	$ 366	62	$ 945
BMS arrangement
Collaboration arrangements
Revenue						$ 222		$ 947
BMS arrangement | Bristol Myers Squibb
Collaboration arrangements
Initial research term						4 years
Number of potential targets included in collaborative agreement | item						10
Extension for research term			1 year
Number of collaboration targets | item						4
License revenue | Bristol Myers Squibb
Collaboration arrangements
Upfront payment recorded		$ 60,100
Expected completion of pre-clinical phase						4 years
Average period for completion of clinical development and commercial launch						8 years 6 months
License revenue | Bristol Myers Squibb | Second, Third, and Fourth Targets Selection
Collaboration arrangements
Target designation payment received		$ 15,000
License revenue | Bristol Myers Squibb | Fifth through Tenth Targets Selection
Collaboration arrangements
Milestone payments to be received upon achievement						$ 16,500
License revenue | Bristol Myers Squibb | Lead Collaboration Target
Collaboration arrangements
Milestone payments to be received upon achievement						254,000
License revenue | Bristol Myers Squibb | Other Selected Targets
Collaboration arrangements
Milestone payments to be received upon achievement						$ 217,000
License revenue | License revenues from related party
Collaboration arrangements
Royalty term after the first commercial sale						10 years