Commitments and contingencies - Accounting Analysis - (Details)	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 28, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) item	Jun. 30, 2015 USD ($)
Allocable arrangement consideration allocation
Committee Obligations: Alliance Joint Steering Committee ("AJSC")	$ 147,000
Committee Obligations: Development Advisory Committee ("DAC")	227,000
License Option and related deliverables	5,743,000
Total allocable arrangement consideration allocations.	79,256,000
Income Statement Related Disclosures [Abstract]
Collaboration revenue		$ 3,720,000	$ 4,884,000	$ 8,550,000	$ 7,460,000
Change in estimated period | item				2
Revenue adjustment on account of change in estimated period of performance		1,200,000
Huntington's Program
Allocable arrangement consideration allocation
Research and development services	15,662,000
Parkinson's Program
Allocable arrangement consideration allocation
Research and development services	6,648,000
Friedreich's Ataxia Program
Allocable arrangement consideration allocation
Research and development services	16,315,000
Spinal Muscular Atrophy Program
Allocable arrangement consideration allocation
Research and development services	32,050,000
Future Program
Allocable arrangement consideration allocation
Research and development services	2,464,000
Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received	65,000,000
Additional consideration, premium over fair value on temporary equity issued	5,000,000
Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received	65,000,000
In-kind services and other funding to be provided by collaborative partner	9,300,000
Income Statement Related Disclosures [Abstract]
Collaboration revenue		3,720,000		$ 8,550,000
Deferred Revenue		$ 47,184,000		$ 47,184,000
Redeemable Convertible Preferred Stock Series B | Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Additional consideration, premium over fair value on temporary equity issued	$ 5,000,000