Collaboration and License Agreements - REGENXBIO Inc. License (Details) - Collaborative Arrangement - ReGenX - USD ($)		1 Months Ended	12 Months Ended
	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Mar. 21, 2014
Collaboration and License Agreements
Up-front payment, agreed-upon amount					$ 2,000,000
Up-front payment, paid	$ 1,000,000	$ 1,000,000
Aggregate potential future milestones					$ 12,250,000
Research and development expenses
Collaboration and License Agreements
Milestone payment			$ 0	$ 250,000
Maintenance fee			$ 50,000	$ 50,000