License Agreements - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	May 31, 2020	Apr. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2017
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, fair value						$ 113,215
Research and development			$ 12,321	$ 16,836	$ 36,681	$ 37,202
REGENXBIO GBA1
Research And Development Arrangement Contract To Perform For Others [Line Items]
Licenses to be expired					7 years
REGENXBIO Option Genes License
Research And Development Arrangement Contract To Perform For Others [Line Items]
Expenses incurred for annual maintenance fee	$ 100
Maintenance Fee	$ 200
License | REGENXBIO GBA1
Research And Development Arrangement Contract To Perform For Others [Line Items]
Stock issued during period, shares, new issues							2,430,000
Common stock, fair value							$ 400
License | REGENXBIO Option Genes License
Research And Development Arrangement Contract To Perform For Others [Line Items]
Additional upfront fee paid		$ 600
Research and development		$ 1,800