Commitments and contingencies - Accounting Analysis - (Details) $ in Thousands		1 Months Ended	12 Months Ended
	Jun. 14, 2019 USD ($)	Feb. 28, 2018 USD ($) item	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Feb. 28, 2015 USD ($)
Income Statement Related Disclosures [Abstract]
Revenue recognized			$ 26,351
Deferred revenue			43,689	$ 70,040
Total payable amount	$ 20,000
Sanofi Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Non-refundable upfront payment received						$ 65,000
Income Statement Related Disclosures [Abstract]
Revenue recognized	28,700			32,600	$ 900
Deferred revenue	48,700
Up-front fee paid on date of termination	10,000
Milestone payable upon achievement	10,000
Sanofi Genzyme | Termination Agreement
Allocable arrangement consideration
Non-refundable upfront payment received	$ 1,000
AbbVie | AbbVie Tau Collaboration Agreement
Allocable arrangement consideration
Non-refundable upfront payment received		$ 69,000
Income Statement Related Disclosures [Abstract]
Revenue recognized			46,300
Number of development options | item		3
AbbVie | AbbVie Tau Collaboration Agreement | Research Services
Income Statement Related Disclosures [Abstract]
Revenue recognized			$ 4,500	$ 11,300	$ 6,900