Commitments and contingencies - Accounting Analysis - (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	Feb. 28, 2015	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
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]
Revenue adjustment on account of change in estimated period of performance										$ 2,400
Collaboration revenue		$ 2,362	$ 3,308	$ 3,720	$ 4,830	$ 4,937	$ 4,937	$ 4,884	$ 2,576	14,220	$ 17,334
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
Additional consideration, premium over fair value on temporary equity issued	5,000
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										14,200	$ 17,300
Deferred Revenue		$ 41,600								$ 41,600
Redeemable Convertible Preferred Stock Series B | Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received	$ 30,000