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Related party transactions (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction
Share Issued value under agreement		$ 95,453
Apple Tree
Related Party Transaction
Expenses for services received from related party		20	$ 0
Versant
Related Party Transaction
Expenses for services received from related party		0	4
venBio
Related Party Transaction
Expenses for services received from related party		0	35
Due to related party	$ 35		$ 35
Series A Preferred Stock
Related Party Transaction
Issuance of stock (in shares)			17,653,333
Share Issued value under agreement			$ 8,787
Series A Preferred Stock \| Apple Tree
Related Party Transaction
Issuance of stock (in shares)			8,000,000
Series A Preferred Stock \| Versant
Related Party Transaction
Issuance of stock (in shares)			10,666,667
Series A Preferred Stock \| venBio
Related Party Transaction
Issuance of stock (in shares)			10,666,667
Series B Preferred Stock
Related Party Transaction
Issuance of stock (in shares)	13,871,948		13,871,948
Share Issued value under agreement			$ 45,271
Series B Preferred Stock \| Apple Tree
Related Party Transaction
Share Issued value under agreement			880,568
Series B Preferred Stock \| Versant
Related Party Transaction
Share Issued value under agreement			722,737
Series B Preferred Stock \| venBio
Related Party Transaction
Share Issued value under agreement			722,737
Atlas Venture Life Science Advisors, LLC
Related Party Transaction
Expenses for services received from related party		22	8
Amount owed to related party			$ 12
Atlas Venture Life Science Advisors, LLC \| Series A Preferred Stock
Related Party Transaction
Issuance of stock (in shares)			10,666,667
Atlas Venture Life Science Advisors, LLC \| Series B Preferred Stock
Related Party Transaction
Share Issued value under agreement			$ 722,737
Atlas Venture Life Science Advisors, LLC \| Research And Development Consulting Services
Related Party Transaction
Expenses for services received from related party		$ 0	$ 23