Commitments and Contingencies - Additional Information (Detail)	1 Months Ended						12 Months Ended
	Dec. 31, 2016 USD ($) ft² Floor	Sep. 30, 2015 USD ($)	Aug. 31, 2015 ft²	Feb. 28, 2015 ft²	Jun. 30, 2014 ft²	Jan. 31, 2014 ft²	Dec. 31, 2016 USD ($) ft² Floor	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Commitments and Contingencies [Line Items]
Letters of credit amount issued		$ 10,700,000
Certificate of deposit purchased		$ 10,700,000
Certificate of deposit maturity date							2017-09
Restricted investments	$ 10,700,000						$ 10,700,000
Other Funding Commitments [Member] | CRO [Member]
Commitments and Contingencies [Line Items]
Expenditures incurred by CROs							8,100,000	$ 8,500,000	$ 4,400,000
EXONDYS 51 [Member]
Commitments and Contingencies [Line Items]
Sales milestone recorded as an in-license right	$ 1,000,000						1,000,000
Collaboration and License Agreement [Member]
Commitments and Contingencies [Line Items]
Development, up-front royalty and sales milestone payments							712,600,000
Development milestone and up-front payments recognized under agreement							$ 47,600,000	200,000	$ 0
Corvallis, Oregon [Member]
Commitments and Contingencies [Line Items]
Lease expiration date							2020-12
Number of lease floors vacated and exited | Floor	1						1
Number of floors | Floor	2						2
Sublease [Member]
Commitments and Contingencies [Line Items]
Lease expiration date	2017-12		2017-09	2016-02		2015-07
Additional rental space under amendment | ft²	7,461		15,077	7,461	10,939	15,077	7,461
Lease termination date					Dec. 31, 2016
Lease Agreements [Member]
Commitments and Contingencies [Line Items]
Total rental space | ft²							88,459
Lease expiration date							2021-01
Letters of credit amount issued	$ 600,000						$ 600,000	$ 600,000