Commitments and contingencies - Accounting Analysis - (Details) $ in Thousands	1 Months Ended			3 Months Ended	9 Months Ended
	Feb. 28, 2018 USD ($) item	Sep. 30, 2016 USD ($)	Feb. 28, 2015 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) item
Allocable arrangement consideration allocation
Committee Obligations: Alliance Joint Steering Committee ("AJSC")			$ 133
Committee Obligations: Development Advisory Committee ("DAC")			207
PD Material Right			7,855
Total allocable arrangement consideration allocations.			74,639
Huntington's Program
Allocable arrangement consideration allocation
Research and development services			14,228
Parkinson's Program
Allocable arrangement consideration allocation
Research and development services			6,040
Friedreich's Ataxia Program
Allocable arrangement consideration allocation
Research and development services			14,821
Spinal Muscular Atrophy Program
Allocable arrangement consideration allocation
Research and development services			29,116
Future Program
Allocable arrangement consideration allocation
Research and development services			2,239
Sanofi 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			4,600
Total allocable arrangement consideration			74,600
Allocable arrangement consideration allocation
Additional option exercise payments | item					0
Income Statement Related Disclosures [Abstract]
Revenue recognized				$ 100	$ 1,300
Deferred revenue				50,400	$ 50,400
Research options | item					1
MRIC
Income Statement Related Disclosures [Abstract]
Payments for shares		$ 2,000
AbbVie | Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received	$ 69,000
Allocable arrangement consideration allocation
Research and development services					$ 34,482
First Development Option Material Right					34,518
Total allocable arrangement consideration allocations.					69,000
Income Statement Related Disclosures [Abstract]
Revenue recognized				2,000	4,300
Deferred revenue				$ 64,700	$ 64,700
Number of development options | item	1
Redeemable Convertible Preferred Stock Series B | Sanofi Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Additional consideration, premium over fair value on temporary equity issued			$ 5,000