License and Collaboration Agreements - Additional Information (Details)								1 Months Ended		12 Months Ended
	May 31, 2016 USD ($)	Jan. 31, 2016 shares	Nov. 30, 2015 USD ($)	Jul. 31, 2015 USD ($)	Jan. 31, 2015 shares	Jan. 31, 2014 shares	Jul. 31, 2013 USD ($) shares	Nov. 30, 2015	Aug. 31, 2015 shares	Dec. 31, 2016 USD ($) Agreement	Dec. 31, 2014 shares
Maximum | Research and Development Expense
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory and net sales milestone payments			$ 7,600,000
REGENXBIO License Agreement related to XLMTM and Pompe [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License fee							$ 300,000
License agreement terms										The Company will also owe REGENXBIO (i) an annual maintenance fee; (ii) up to $8.85 million in combined milestone fees per licensed product related to XLMTM and up to $8.85 million in combined milestone fees per licensed product related to Pompe disease, a small portion of which may be paid in the form of shares of the Company’s common stock; (iii) mid to high single digit royalty percentages on net sales of licensed products and (iv) mid- single digit to low twenties royalty percentages of any sublicense fees received by the Company from sublicensees for the licensed patent rights.
REGENXBIO License Agreement related to XLMTM | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone fees							8,850,000
REGENXBIO License Agreement related to Pompe | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone fees							$ 8,850,000
REGENXBIO License Agreement Related to Crigler-Najjar Syndrome [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement terms										The Company will also owe REGENXBIO (i) an annual maintenance fee; (ii) up to $7.6 million in combined development and regulatory milestone fees per licensed product; (iii) mid-single digit to low teens royalty percentages on net sales of licensed products sold by the Company, its affiliates and sublicensees and (iv) a low twenties percentage of certain sublicense fees received by the Company from sublicensees for the licensed products and certain fees received from the sale or transfer of specified rights related to a licensed product.
Upfront fee of consideration for licensed rights			200,000
Additional upfront fee paid			400,000
License agreement expiration period								10 years
REGENXBIO License Agreement Related to CPVT
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement terms										The Company will also owe REGENXBIO (i) an annual maintenance fee for each covered indication; (ii) up to $8.8 million in combined development and regulatory milestone fees for each indication and each licensed product; (iii) up to $45.0 million in combined commercial milestone fees based on various annual aggregate net sales thresholds; (iv) mid-single digit to low teens royalty percentages on net sales of licensed products sold by the Company, its affiliates and sublicensees and (v) a low twenties percentage of any sublicense fees received by the Company from sublicensees for the licensed products and certain fees it received from the sale or transfer of specified rights related to a licensed product.
Upfront fee of consideration for licensed rights			1,000,000
Additional upfront fee paid			500,000
License agreement expiration period								10 years
REGENXBIO License Agreement Related to CPVT | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone fees			45,000,000
Development, regulatory and net sales milestone payments			$ 8,800,000
Genethon Collaborative Development Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock vested | shares		87,466			87,465	87,465
University of Florida License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement expiration period				10 years
University of Florida License Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory and net sales milestone payments				$ 1,200,000
University of Pennsylvania License and Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement terms										The Company is obligated to pay the University of Pennsylvania (i) up to an aggregate of $6.0 million for preclinical development activities agreed upon by both parties, of which $5.7 million had been paid as of December 31, 2016, subject to adjustment based on the work plan; (ii) up to an aggregate of $13.7 million in development, regulatory and net sales milestone payments for the first licensed product; (iii) low to mid-single-digit royalty percentages on tiered annual net sales of the licensed products sold by the Company, its affiliates or sublicensees and (iv) mid-single-digit to low double-digit percentages of any sublicense fees received by it from third parties for the grant of sublicenses to any licensed patent rights.
Upfront fee of consideration for licensed rights	$ 500,000
Development, regulatory and net sales milestone payments	$ 13,700,000
License agreement expiration period	10 years
Preclinical development activities fees										$ 5,700,000
License agreement expiration terms										The agreement will continue on a country-by-country basis and expire upon the later of the expiration of the last valid claim of the licensed patent rights that covers the exploitation of such licensed patent rights in such country, or ten years after first commercial sale of such licensed product in such country.
License agreement termination description										The Company may terminate the agreement upon 60 days’ prior written notice.
License agreement termination period	60 days
Number of other intellectual property license agreements | Agreement										2
University of Pennsylvania License and Collaboration Agreement | Research and Development Expense
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fee of consideration for licensed rights										$ 100,000
Preclinical development activities fees	$ 4,500,000
University of Pennsylvania License and Collaboration Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Preclinical development activities fees	$ 6,000,000
Aggregate contingent future development and commercialization milestones payment										$ 1,900,000
Percentage of royalty on product sales										1.00%
The University Of Pennsylvania Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone fees										$ 700,000
The University Of Pennsylvania Collaboration Agreement | REGENXBIO
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License fee										$ 400,000
Common Stock | FSM
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Shares issued | shares									51,968
Common Stock | Cardiogen | FSM
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement shares converted in connection with business acquisition | shares									425,000
Common Stock | REGENXBIO License Agreement related to XLMTM and Pompe [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Shares issued | shares							50,228
Common Stock | Genethon Collaborative Development Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Shares issued | shares						262,396					262,396