Related Party Transactions - Summary of Related Party Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Related Parties:
Related party expenses	$ 2,498	$ 1,459	$ 6,158	$ 4,923
REGENXBIO, Inc [Member]
Related Parties:
Related party expenses	1,000	1,445	2,000	4,881
University of Pennsylvania [Member]
Related Parties:
Related party expenses	$ 1,498		$ 4,158
Fidelity Biosciences [Member]
Related Parties:
Related party expenses		$ 14		$ 42