Related Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
REGENXBIO, Inc [Member]
Related Party Transaction [Line Items]
Cash payment to related party	$ 1,000,000	$ 2,083,000	$ 2,750,000	$ 5,896,000
Amounts due to related party	0		0		$ 750,000
University of Pennsylvania [Member]
Related Party Transaction [Line Items]
Cash payment to related party	1,000,000	0	4,137,000	0
Amounts due to related party	21,000		21,000		0
Fidelity Biosciences [Member]
Related Party Transaction [Line Items]
Cash payment to related party	0	$ 0	1,000	$ 38,000
Amounts due to related party	$ 0		$ 0		$ 0