Commitments and Contingencies - Additional Information (Detail)	1 Months Ended						12 Months Ended
	Dec. 31, 2016 USD ($) ft²	Sep. 30, 2015 USD ($)	Aug. 31, 2015 ft²	Feb. 28, 2015 ft²	Jun. 30, 2014 ft²	Jan. 31, 2014 ft²	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 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
Restricted investment maturity date							2017-09
EXONDYS 51 [Member]
Commitments and Contingencies [Line Items]
Sales milestone recorded as an in-licensed rights	$ 1,000,000						$ 6,600,000	1,000,000
Collaboration and License Agreement [Member]
Commitments and Contingencies [Line Items]
Development milestone and up-front payments							22,000,000	47,600,000	$ 200,000
Collaboration and License Agreement [Member] | Maximum [Member]
Commitments and Contingencies [Line Items]
Development, up-front royalty and sales milestone payments							808,500,000
BioMarin Pharmaceutical, Inc. [Member]
Commitments and Contingencies [Line Items]
Royalty payments							4,700,000
CRO [Member]
Commitments and Contingencies [Line Items]
Cancellable future commitments							52,900,000
CRO [Member] | Other Funding Commitments [Member]
Commitments and Contingencies [Line Items]
Expenditures incurred by CROs							$ 13,900,000	$ 8,100,000	$ 8,500,000
Corvallis, Oregon [Member]
Commitments and Contingencies [Line Items]
Lease expiration date								2020-12
Sublease [Member]
Commitments and Contingencies [Line Items]
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 expired date	2017-12		2017-03	2016-02		2015-07
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