Commitments and contingencies - Accounting Analysis - (Details) $ in Millions		1 Months Ended		3 Months Ended		6 Months Ended
	Jun. 14, 2019 USD ($)	Feb. 28, 2018 USD ($) item	Feb. 28, 2015 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) item	Jun. 30, 2019 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	$ 30.2	$ 0.0	$ 31.6
Deferred revenue	48.7			10.0		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
Up-front fee paid on date of termination	$ 10.0
AbbVie | AbbVie Tau Collaboration Agreement
Allocable arrangement consideration
Non-refundable upfront payment received		$ 69.0
Income Statement Related Disclosures [Abstract]
Revenue recognized				1.1	$ 3.2	3.5	$ 4.9
Deferred revenue				$ 47.2		$ 47.2
Number of development options | item		3				3