Related Party Transactions - Additional Information (Detail) - USD ($)		1 Months Ended				4 Months Ended	9 Months Ended	12 Months Ended
	Jan. 31, 2015	Aug. 31, 2015	May 31, 2015	Mar. 31, 2015	Sep. 30, 2014	Apr. 30, 2015	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jul. 31, 2014
Related Party Transaction [Line Items]
Agreement expiry date										Dec. 31, 2014
License revenue from license agreements								$ 4,303,000	$ 5,025,000	$ 4,355,000
Preferred stock, par value								$ 0.0001	$ 0.0001
Rent expense								$ 1,800,000	$ 400,000
Maximum [Member]
Related Party Transaction [Line Items]
Rent expense										100,000
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			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			1,300,000
Dimension Therapeutics, Inc. [Member]
Related Party Transaction [Line Items]
Annual maintenance fees				$ 100,000
Research and development expense reimbursements earned										6,200,000
Net reimbursable costs										0
Extension fee paid by Dimension for extension of option					$ 200,000
Option fee payable upon exercise				1,000,000
Milestone payment received from option and license agreement		$ 1,000,000	$ 1,000,000	$ 9,000,000					1,000,000
License revenue from license agreements							2,000,000			200,000
Amount received for purchase of materials from related party										200,000
Gain on disposal of the material to related party								$ 200,000		200,000
Dimension Therapeutics, Inc. [Member] | Maximum [Member]
Related Party Transaction [Line Items]
Gains related to delivery of purchased material							$ 100,000			100,000
Dimension Therapeutics, Inc. [Member] | Commercial License Agreement [Member]
Related Party Transaction [Line Items]
License agreement initiation date								Oct. 31, 2013
Dimension Therapeutics, Inc. [Member] | Commercial License Agreement [Member] | Maximum [Member]
Related Party Transaction [Line Items]
Annual maintenance fees								$ 100,000
Penn [Member]
Related Party Transaction [Line Items]
Research and development expense to Penn										6,200
FoxKiser,LLP [Member]
Related Party Transaction [Line Items]
Rent expense						$ 100,000
Lease expiration date	Apr. 30, 2015
FoxKiser,LLP [Member] | Promissory Notes [Member]
Related Party Transaction [Line Items]
Accounts receivable from related party											$ 1,800,000
Estimated market rate								15.00%
Aggregate discount										100,000
FoxKiser,LLP [Member] | Second Promissory Notes [Member]
Related Party Transaction [Line Items]
Accounts receivable from related party					$ 600,000
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 percent per month, compounding monthly.
Accrued interest compounding monthly on amount outstanding								1.50%
FoxKiser,LLP [Member] | Service Agreements [Member] | Series C Preferred Stock [Member]
Related Party Transaction [Line Items]
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] | Promissory Notes [Member] | Series C Preferred Stock [Member]
Related Party Transaction [Line Items]
Issuance of convertible preferred stock for the conversion of outstanding related party debt	$ 3,800,000
FoxKiser,LLP [Member] | Service Agreements [Member] | Second Promissory Notes [Member] | Series C Preferred Stock [Member]
Related Party Transaction [Line Items]
Issuance of convertible preferred stock for the conversion of outstanding related party debt	2,400,000
FoxKiser,LLP [Member] | Service Agreements [Member] | Primary Promissory Notes [Member] | Series C Preferred Stock [Member]
Related Party Transaction [Line Items]
Issuance of convertible preferred stock for the conversion of outstanding related party debt	1,400,000
FoxKiser,LLP [Member] | Service Agreements [Member] | Research and Development Costs [Member]
Related Party Transaction [Line Items]
Related party transaction expense								$ 1,000,000	200,000	1,300,000
FoxKiser,LLP [Member] | Service Agreements [Member] | General and Administrative [Member]
Related Party Transaction [Line Items]
Related party transaction expense									200,000	1,700,000
FoxKiser,LLP [Member] | Maximum [Member]
Related Party Transaction [Line Items]
Monthly lease payments	$ 100,000
FoxKiser,LLP [Member] | Maximum [Member] | Service Agreements [Member] | Promissory Notes [Member]
Related Party Transaction [Line Items]
Interest expense									100,000	300,000
FoxKiser,LLP [Member] | Maximum [Member] | Service Agreements [Member] | Preferred Stock [Member]
Related Party Transaction [Line Items]
Amount owed under service agreement											$ 2,000,000
Chief Scientific Advisor [Member]
Related Party Transaction [Line Items]
Related party transaction expenses from transactions with related party advisory fees								$ 300,000	$ 200,000
Shares, Granted								211,600,000
Chief Scientific Advisor [Member] | Maximum [Member]
Related Party Transaction [Line Items]
Related party transaction expenses from transactions with related party advisory fees										$ 100,000