Collaboration arrangements and concentration of credit risk - BMS collaboration - Narrative (Details) $ in Thousands		1 Months Ended	3 Months Ended		12 Months Ended
	May 21, 2015 USD ($)	Feb. 28, 2019	Mar. 31, 2020 USD ($) item	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Collaboration arrangements
Revenue			$ 104	$ 1,136
License revenues from related party
Collaboration arrangements
Revenue			47	557
Collaboration revenues from related party
Collaboration arrangements
Revenue			57	$ 579
BMS arrangement
Collaboration arrangements
Revenue			$ 258		$ 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