Commitments and contingencies - Accounting Analysis - (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended
	Sep. 30, 2016	Feb. 28, 2015	Mar. 31, 2017	Mar. 31, 2016
Allocable arrangement consideration allocation
Committee Obligations: Alliance Joint Steering Committee ("AJSC")		$ 147
Committee Obligations: Development Advisory Committee ("DAC")		227
License Option and related deliverables		5,743
Total allocable arrangement consideration allocations.		79,256
Income Statement Related Disclosures [Abstract]
Collaboration revenue			$ 1,464	$ 4,830
Huntington's Program
Allocable arrangement consideration allocation
Research and development services		15,662
Parkinson's Program
Allocable arrangement consideration allocation
Research and development services		6,648
Friedreich's Ataxia Program
Allocable arrangement consideration allocation
Research and development services		16,315
Spinal Muscular Atrophy Program
Allocable arrangement consideration allocation
Research and development services		32,050
Future Program
Allocable arrangement consideration allocation
Research and development services		2,464
Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received		65,000
Additional consideration, premium over fair value on temporary equity issued		5,000
Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received		65,000
In-kind services and other funding to be provided by collaborative partner		9,300
Total allocable arrangement consideration		79,300
Income Statement Related Disclosures [Abstract]
Collaboration revenue			1,500
Deferred Revenue			$ 40,200
MRIC
Income Statement Related Disclosures [Abstract]
Payments for shares	$ 2,000
Redeemable Convertible Preferred Stock Series B | Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Additional consideration, premium over fair value on temporary equity issued		$ 5,000