Commitments and Contingencies - Additional Information (Detail)	1 Months Ended						12 Months Ended
	Oct. 31, 2018 USD ($) Suite	Sep. 30, 2018 USD ($)	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($) Payment Month Quarter	Dec. 31, 2016 USD ($) ft²	Aug. 31, 2015 ft²	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) ft²
Settled Litigation [Member]
Commitments and Contingencies [Line Items]
Settlement date							September 4, 2018
EXONDYS 51 [Member]
Commitments and Contingencies [Line Items]
Sales milestone recorded as an in-licensed rights assets					$ 1,000,000		$ 0	$ 6,600,000	$ 1,000,000
Collaboration and License Agreement [Member]
Commitments and Contingencies [Line Items]
Development milestone and up-front payments							142,400,000	22,000,000	47,600,000
Collaboration and License Agreement [Member] | Maximum [Member]
Commitments and Contingencies [Line Items]
Development, regulatory, up-front royalty and sales milestone payments							378,800,000
BioMarin Pharmaceutical, Inc. [Member]
Commitments and Contingencies [Line Items]
Royalty expense							15,100,000	4,700,000
Sales milestone recorded as an in-licensed rights assets							0
CRO [Member]
Commitments and Contingencies [Line Items]
Cancellable future commitments							71,600,000
CRO [Member] | Other Funding Commitments [Member]
Commitments and Contingencies [Line Items]
Expenditures incurred by CROs							$ 19,600,000	13,900,000	$ 8,500,000
Brammer [Member] | Manufacturing Agreement [Member]
Commitments and Contingencies [Line Items]
Advance upfront fee to be paid on execution of agreements				$ 20,000,000
Number of non-refundable process equipment fee payments | Payment				2
Required non refundable payments at each time of procurement of equipment				$ 5,000,000
Quarterly capacity access fee payments		$ 5,000,000	$ 5,000,000	$ 10,000,000
Portion of advance payment applied as credit to quarterly capacity access fees							one-tenth
Quarterly capacity access fees due and payable beginning date				Jul. 01, 2019
Quarterly capacity access fees due and payable ending date				Dec. 31, 2021
Net capacity access fee				$ 8,000,000
Manufacturing agreement term				6 years
Successive renewal term of manufacturing agreement				2 years
Minimum number of months prior to expiration of term for renewal | Month				24
Maximum required additional quarters for remittance of capacity access fees before terminate agreement | Quarter				8
Cash payment under agreements							$ 40,000,000
Advance upfront fee paid on agreements				$ 20,000,000
Process equipment fee paid	$ 5,000,000		$ 5,000,000
Brammer [Member] | Manufacturing Agreement [Member] | Other Current Assets [Member]
Commitments and Contingencies [Line Items]
Cash payment under agreements							5,000,000
Brammer [Member] | Manufacturing Agreement [Member] | Other Non-current Asset [Member]
Commitments and Contingencies [Line Items]
Cash payment under agreements							$ 35,000,000
Paragon [Member] | Paragon Agreement [Member]
Commitments and Contingencies [Line Items]
Number of dedicated clean room suites | Suite	2
Number of additional clean room suites available on optional reserve | Suite	2
Usage fees per annum per clean room suite	$ 1,000,000
Agreement expiration date	Dec. 31, 2025
Agreement expiration description							The term of the Paragon Collaboration Agreement will continue until the earlier of: (i) the latest expiration date of any commercial supply agreement in effect, or (ii) December 31, 2025. The Company has the ability to terminate the Paragon Collaboration Agreement prior to expiration, subject to potential additional financial consideration.
Paragon [Member] | Paragon Agreement [Member] | Other Non-current Asset [Member]
Commitments and Contingencies [Line Items]
Clean room suite reservation fees payments	$ 26,000,000
Paragon [Member] | Paragon Agreement [Member] | Maximum [Member]
Commitments and Contingencies [Line Items]
Cean room suite reservation fees	$ 48,000,000
Percentage of credit on clean room use fee if threshold met	100.00%
Aldevron LLC [Member] | Clinical and Commercial Supply Agreement [Member]
Commitments and Contingencies [Line Items]
Agreement expiration date							Dec. 31, 2019
Agreement expiration description							The Company has an option to extend the CCSA to December 31, 2020 (the “2020 Option”) and other option to extend the term of the CCSA for an additional year to December 31, 2021 (the “2021 Renewal Right”).
Percentage of prepayments credited back equal to batch invoice amount							50.00%
Aldevron LLC [Member] | Clinical and Commercial Supply Agreement [Member] | Other Current Assets [Member]
Commitments and Contingencies [Line Items]
Prepayments to extend agreement term							$ 10,000,000
Aldevron LLC [Member] | Clinical and Commercial Supply Agreement [Member] | Maximum [Member]
Commitments and Contingencies [Line Items]
Advance upfront fee to be paid on execution of agreements							$ 60,000,000
Dublin [Member]
Commitments and Contingencies [Line Items]
Lease expiration date							2019-11
Lease agreement entered date							2018-12
Additional office and lab Space on lease | ft²							22,600
Columbus [Member]
Commitments and Contingencies [Line Items]
Lease expiration date							2026-06
Lease agreement entered date							2018-12
Additional office and lab Space on lease | ft²							77,679
Corvallis [Member]
Commitments and Contingencies [Line Items]
Lease expiration date							2020-12
Lease terminated date month year							2018-06
Sublease [Member] | Third Party One [Member]
Commitments and Contingencies [Line Items]
Additional rental space under amendment | ft²						15,077
Lease agreement entered date						2015-08
Lease expired date						2017-03
Sublease [Member] | Third Party Two [Member]
Commitments and Contingencies [Line Items]
Additional rental space under amendment | ft²					7,461				7,461
Lease agreement entered date					2016-12
Lease expired date					2017-12
Sublease [Member] | Andover [Member]
Commitments and Contingencies [Line Items]
Additional rental space under amendment | ft²							23,102
Lease agreement entered date description							March and July of 2018
Lease expiration date							2020-12
Sublease [Member] | Burlington [Member]
Commitments and Contingencies [Line Items]
Additional rental space under amendment | ft²							44,740
Lease expiration date							2022-01
Lease agreement entered date							2018-11
Lease Agreements [Member]
Commitments and Contingencies [Line Items]
Lease expiration date							Sep. 30, 2025
Total rental space | ft²							163,842
Letters of credit amount issued							$ 1,000,000	$ 1,000,000