Commitments and contingencies - Accounting Analysis - (Details) - Sanofi Genzyme - USD ($) $ in Millions		1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 14, 2019	Feb. 28, 2015	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Huntington's Program
Income Statement Related Disclosures [Abstract]
Deferred revenue	$ 10.0
Milestone payable upon achievement	10.0
Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received		$ 65.0
Additional consideration, premium over fair value on temporary equity issued		$ 5.0
Income Statement Related Disclosures [Abstract]
Revenue recognized	28.7		$ 0.2	$ 0.1	$ 31.8	$ 1.3
Deferred revenue	48.7
Up-front fee paid on date of termination	10.0
Milestone payable upon achievement	10.0
Total payable amount	20.0
Termination Agreement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received	$ 1.0
Income Statement Related Disclosures [Abstract]
Deferred revenue			$ 10.0		$ 10.0