Related Party Transactions - Additional Information (Detail) - USD ($)		4 Months Ended	9 Months Ended	12 Months Ended
	Jan. 31, 2015	Apr. 30, 2015	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Related Party Transaction [Line Items]
License revenue from license agreements				$ 10,385,000	$ 4,303,000	$ 5,025,000
Preferred stock, par value				$ 0.0001	$ 0.0001
Rent expense				$ 1,800,000	$ 1,800,000	400,000
The Trustees of the University of Pennsylvania [Member] | Manufacturing of Reagents for Sale [Member]
Related Party Transaction [Line Items]
Related party transaction expense			$ 100,000
The Trustees of the University of Pennsylvania [Member] | Research and Development Services [Member]
Related Party Transaction [Line Items]
Related party transaction expense			5,100,000
Dimension Therapeutics Inc
Related Party Transaction [Line Items]
License revenue from license agreements			2,000,000
Amount received for purchase of materials from related party							$ 200,000
Gain on disposal of the material to related party							$ 200,000
Gains related to delivery of purchased material			$ 100,000
FoxKiser,LLP [Member]
Related Party Transaction [Line Items]
Interest expense	$ 100
Monthly lease payments	$ 100,000
Rent expense		$ 100,000
Lease expiration date	Apr. 30, 2015
FoxKiser,LLP [Member] | Series C Preferred Stock
Related Party Transaction [Line Items]
Issuance of convertible preferred stock for the conversion of outstanding related party debt	$ 3,800,000
Issuance of convertible preferred stock for the conversion of outstanding related party debt, Shares	585,578
Preferred stock, par value	$ 6.477
FoxKiser,LLP [Member] | Service Agreements [Member]
Related Party Transaction [Line Items]
Related party transaction, description				The amounts outstanding to FOXKISER under the services agreement in excess of 30 days from their due date accrued interest at 1.5% per month, compounding monthly.
Accrued interest compounding monthly on amount outstanding				1.50%
FoxKiser,LLP [Member] | Service Agreements [Member] | Research and Development Costs [Member]
Related Party Transaction [Line Items]
Related party transaction expense				$ 1,500,000	1,000,000	200,000
FoxKiser,LLP [Member] | Service Agreements [Member] | General and Administrative [Member]
Related Party Transaction [Line Items]
Related party transaction expense						200,000
Chief Scientific Advisor [Member]
Related Party Transaction [Line Items]
Related party transaction expenses from transactions with related party advisory fees				$ 100,000	$ 300,000	$ 200,000
Shares, Granted						211,600,000	211,600,000