Commitments and contingencies - Significant Agreements - (Details) - Sanofi Genzyme - USD ($) $ in Millions		1 Months Ended
	Jun. 14, 2019	Feb. 28, 2015
Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non-refundable upfront payment received		$ 65.0
Premium over fair value on temporary equity issued		$ 5.0
Up-front fee paid on date of termination	$ 10.0
Milestone payable upon achievement	10.0
Termination Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non-refundable upfront payment received	1.0
Huntington's Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payable upon achievement	$ 10.0
Redeemable Convertible Preferred Stock Series B | Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Temporary equity issued (in shares)		10,000,000
Temporary equity issued		$ 30.0
Fair value of temporary equity issued		$ 25.0