Commitments and contingencies - Accounting Analysis - (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Feb. 28, 2018 USD ($)	Feb. 28, 2015 USD ($)	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)
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
Income Statement Related Disclosures [Abstract]
Revenue recognized				$ 7,800
Number of accounting units | item				2
Sanofi Genzyme | Spinal Muscular Atrophy Program
Income Statement Related Disclosures [Abstract]
Revenue adjustment due to development deprioritization				$ 9,500
Sanofi Genzyme | PD Material Right Program
Income Statement Related Disclosures [Abstract]
Revenue adjustment due to development deprioritization				2,100
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			$ 700	$ 10,100	$ 14,200
Deferred revenue			50,900
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	$ 6,900
Deferred revenue			$ 62,100
Redeemable Convertible Preferred Stock Series B | Sanofi Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received		$ 30,000