Commitments and contingencies - Accounting Analysis - (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2016	Feb. 28, 2015	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 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			$ 1,148		$ 3,308	$ 3,790	$ 11,858
Revenue adjustment on account of change in estimated period of performance				$ 500
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
Income Statement Related Disclosures [Abstract]
Revenue adjustment on account of change in estimated period of performance								$ 2,400
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			1,100			3,800
Deferred Revenue			$ 37,900			$ 37,900
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