Commitments and contingencies - Accounting Analysis - (Details) $ in Millions		1 Months Ended	3 Months Ended
	Jun. 14, 2019 USD ($)	Feb. 28, 2015 USD ($)	Mar. 31, 2020 USD ($) item	Mar. 31, 2019 USD ($)	Feb. 28, 2018 USD ($)
Sanofi Genzyme | Huntington's Program
Income Statement Related Disclosures [Abstract]
Deferred revenue	$ 10.0
Sanofi Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Non-refundable upfront payment received		$ 65.0
Additional consideration, premium over fair value on temporary equity issued		$ 5.0
Income Statement Related Disclosures [Abstract]
Revenue recognized			$ 0.0	$ 1.4
Deferred revenue	48.7		10.0
Milestone payable upon achievement	10.0
Total payable amount	20.0
Sanofi Genzyme | Termination Agreement
Allocable arrangement consideration
Non-refundable upfront payment received	1.0
Income Statement Related Disclosures [Abstract]
Revenue recognized	28.7
Deferred revenue	10.0
Up-front fee paid on date of termination	10.0
Milestone payable upon achievement	$ 15.0
AbbVie | AbbVie Tau Collaboration Agreement
Allocable arrangement consideration
Non-refundable upfront payment received					$ 69.0
Income Statement Related Disclosures [Abstract]
Revenue recognized			2.4	$ 1.7
Deferred revenue			$ 48.3
Number of development options | item			3