Significant agreements - AstraZeneca Collaboration Agreement (Details) $ in Thousands		1 Months Ended			3 Months Ended								12 Months Ended
	May 31, 2019 employee	May 31, 2018 USD ($)	Aug. 31, 2017 USD ($)	Nov. 30, 2016 USD ($) employee item	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Oct. 31, 2017 USD ($)
Significant agreements.
Transaction price			$ 14,200
Collaboration revenues					$ 5,281	$ 614	$ 1,522	$ 6,384	$ 1,057	$ 1,610	$ 1,661	$ 2,808	$ 13,801	$ 7,136	$ 2,060
Deferred revenue					5,657				14,635				5,657	14,635	14,467
AstraZeneca
Significant agreements.
Biological Targets | item				6
Term (in years)			3 years
Term, Bicycle Research Team (in years)				1 year
Term, AZ Research (in years)				2 years
Number of FTE | employee				2
Collaboration revenues													1,683	1,386	890
Deferred revenue					4,913				4,727				4,913	4,727
AstraZeneca | Target One
Significant agreements.
Transaction price				$ 600												$ 2,000
AstraZeneca | Target Two
Significant agreements.
Transaction price				600
AstraZeneca | Target Three Research License and Related Services
Significant agreements.
Collaboration revenues													1,500	400	0
AstraZeneca | Development Milestone
Significant agreements.
Transaction price		$ 700
AstraZeneca | Development Milestone | Development milestone
Significant agreements.
Customer option payment				29,000
AstraZeneca | Regulatory Milestone | Regulatory milestone
Significant agreements.
Customer option payment				23,000
AstraZeneca | Commercial milestone | Commercial milestone
Significant agreements.
Customer option payment				$ 110,000
AstraZeneca | Minimum
Significant agreements.
Term extension (in years)				12 months
AstraZeneca | Maximum
Significant agreements.
Term extension (in years)				15 months
AstraZeneca | 2016 Collaboration Agreement
Significant agreements.
Biological Targets | item				2
Option fee for development and exploitation rights				$ 8,000
Transaction price				$ 1,200
Research term related to research license and related services				1 year
Deferred revenue									$ 8,000					8,000	0
AstraZeneca | 2016 Collaboration Agreement | Target One and Target Two Research License and Related Services
Significant agreements.
Collaboration revenues													200	$ 1,000	$ 900
AstraZeneca | 2016 Collaboration Agreement | Commercialization license per candidate
Significant agreements.
Customer option payment				$ 8,000
AstraZeneca | May 2018 Option Exercise
Significant agreements.
Biological Targets | item				4
Option fee for development and exploitation rights		5,000		$ 5,000
Number of FTE | employee	2
Transaction price					5,650		$ 6,300						5,650
AstraZeneca | May 2018 Option Exercise | Target Three Research License and Related Services
Significant agreements.
Transaction price					650								650
AstraZeneca | May 2018 Option Exercise | Target 3 Material Right
Significant agreements.
Transaction price					1,504								1,504
AstraZeneca | May 2018 Option Exercise | Target 4 Material Right
Significant agreements.
Transaction price					1,204								1,204
AstraZeneca | May 2018 Option Exercise | Target 5 Material Right
Significant agreements.
Transaction price					1,165								1,165
AstraZeneca | May 2018 Option Exercise | Target 6 Material Right
Significant agreements.
Transaction price					$ 1,127								$ 1,127
AstraZeneca | May 2018 Option Exercise | Commercialization license per candidate
Significant agreements.
Customer option payment		$ 8,000