Commitments and contingencies - Significant Agreements - (Details) - Genzyme - USD ($)	1 Months Ended		3 Months Ended
	Oct. 31, 2017	Feb. 28, 2015	Mar. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received		$ 65,000,000
Amount of reduction in milestone or royalty payments to be made by collaborative partner for each program specified		10,000,000
Per Milestone, maximum milestone payments to be received from collaborative partner	$ 540,000,000
Regulatory approval milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	220,000,000
Regulatory approval milestone | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	40,000,000
Regulatory approval milestone | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	50,000,000
Commercial Milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	320,000,000
Commercial Milestone | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	50,000,000
Commercial Milestone | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	100,000,000
Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received		65,000,000
Milestone payments no longer receivable			$ 0
Collaborative Arrangement | Regulatory and commercial milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments no longer receivable	$ 105,000,000
Spinal Muscular Atrophy Program
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum potential in-kind services to be provided by collaborative partner		5,000,000
Option exercise payment required from collaborative partner for each program specified		30,000,000
Split Territory Program
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Option exercise payment required from collaborative partner for each program specified		20,000,000
Redeemable Convertible Preferred Stock Series B
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received		$ 30,000,000
Temporary equity issued (in shares)		10,000,000
Fair value of temporary equity issued		$ 25,000,000
Premium over fair value on temporary equity issued		5,000,000
Redeemable Convertible Preferred Stock Series B | Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Premium over fair value on temporary equity issued		$ 5,000,000