Revenue - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Jan. 01, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Feb. 29, 2020	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue recognized		$ 181	$ 148	$ 675	$ 795
Licensing agreements
Disaggregation of Revenue [Line Items]
Revenue recognized		181	148	675	795
Licensing agreements | Sarepta
Disaggregation of Revenue [Line Items]
Research and development costs, percentage of costs responsible	45.00%
Licensing agreements | EuroFarma
Disaggregation of Revenue [Line Items]
Deferred revenue, upfront payment							$ 2,500
Deferred revenue, enrollment milestone						$ 1,000
Revenue recognized related to upfront payment		200	$ 100	700	500
Revenue entitled to receive upon achieving patient enrollment targets				2,750
Revenue entitled to receive upon achieving cumulative net sales targets				21,400
Cumulative net sales target threshold				100,000
Incremental cumulative net sales target threshold				$ 100,000
Licensing agreements | Sarepta
Disaggregation of Revenue [Line Items]
Research and development costs, percentage of costs responsible	55.00%
Research and development costs, amount borne by party incurred, threshold percentage of budged amount	110.00%
Revenue recognized		$ 0			$ 300