License and Collaboration Agreements - Additional Information (Detail) - USD ($)		1 Months Ended		12 Months Ended
	Oct. 03, 2016	Jun. 30, 2016	Apr. 30, 2013	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
EXONDYS 51 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sales milestone payment recorded as an in-license right				$ 1,000,000
Collaboration and License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of budgeted research and development cost allocation	45.00%
Maximum percentage of budgeted research and development cost	110.00%
Development milestone and up-front payments				47,600,000	$ 200,000	$ 0
Summit [Member] | Collaboration and License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront milestone payment	$ 40,000,000
One-time option fee	$ 10,000,000
Percentage of budgeted research and development cost allocation	55.00%
Development milestone and up-front payments				40,000,000
Summit [Member] | Collaboration and License Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional payments upon successful achievement	$ 192,000,000
Development and regulatory milestone payments	290,000,000
Sales milestone payments	330,000,000
Regulatory milestone payments upon exercise of option	7,000,000
Sales milestone payments upon exercise of option	$ 82,500,000
University of Western Australia [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestone and up-front payments				7,600,000	$ 200,000	$ 0
Up-front payment to purchase future royalties			$ 30,000,000
Up-front payment upon execution of amendment		$ 7,000,000
Royalty payments				0
University of Western Australia [Member] | EXONDYS 51 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sales milestone payment recorded as an in-license right				$ 1,000,000
University of Western Australia [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional payments upon successful achievement		$ 20,000,000	20,000,000
University of Western Australia [Member] | Maximum [Member] | EXONDYS 51 and Up to Five Additional Products [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments to UWA on successful achievement of certain development and regulatory milestones			$ 6,000,000