License and Collaboration Agreements - Additional Information (Detail)				1 Months Ended				3 Months Ended								12 Months Ended
	Oct. 15, 2018 USD ($) Milestone shares	Oct. 08, 2018 USD ($)	May 03, 2018 USD ($)	Jul. 31, 2017 USD ($)	Oct. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Apr. 30, 2013 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Milestone	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Aug. 08, 2018 USD ($) shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment								$ 3,300,000								$ 3,300,000
Research and development expense								146,207,000	$ 86,584,000	$ 122,848,000	$ 46,204,000	$ 44,441,000	$ 34,239,000	$ 58,908,000	$ 29,119,000	401,843,000	$ 166,707,000	$ 188,272,000
Upfront fee to be paid on execution of agreements				$ 35,000,000
Intangible asset, net								11,574,000				14,355,000				11,574,000	14,355,000
Settlement and license charges													25,588,000	2,839,000			28,400,000
Intangible asset amortization expense								216,000	$ 216,000	$ 217,000	$ 216,000	216,000	$ 780,000	$ 29,000	$ 28,000	865,000	1,053,000	29,000
Intangible asset, accumulated amortization								3,852,000				4,145,000				3,852,000	4,145,000
EXONDYS 51 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sales milestone payment recorded as an in-license right								0				6,600,000				0	6,600,000	1,000,000
Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Regulatory and sales milestone payments				$ 65,000,000
Percentage of royalty payments				8.00%
Intangible asset amortization expense																		100,000
Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of royalty payments				4.00%
Lysogne [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Recognized loss related to fair market value of equity investment																$ 200,000
Lacerta Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of development milestone | Milestone																0
Research and development expense																$ 0
BioMarin Pharmaceutical, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front cash payment under agreements				$ 35,000,000
Regulatory and sales milestone payments																0
Intangible asset, net								6,600,000				$ 6,600,000				6,600,000	6,600,000
Intangible asset amortization expense																900,000	1,000,000
Royalty expense																15,100,000	4,700,000
Sales milestone payment recorded as an in-license right								0								0
BioMarin Pharmaceutical, Inc. [Member] | U.S. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payment expiry period				2023-12
BioMarin Pharmaceutical, Inc. [Member] | EU [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payment expiry period				2024-09
University of Western Australia [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty expense																0	0	0
Development milestone and up-front payments																0	0	7,600,000
Up-front payment to purchase future royalties							$ 30,000,000
Up-front payment upon execution of amendment						$ 7,000,000
Purchase option upfront payment for future royalties						23,000,000
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				1,000,000								1,000,000
Intangible asset, accumulated amortization								300,000								300,000
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
Equity Investment Agreement [Member] | Lysogne [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock purchased,shares | shares	950,606
Percentage of common stock outstanding	8.00%
Upfront license fee, option fee, equity investment and certain development activities reimbursement payable on execution of agreement	$ 28,000,000
Upfront license fee, option fee and certain development activities reimbursement payable on execution of agreement	$ 26,100,000
Number of development milestone | Milestone	1
Accrued research and development expense								18,700,000								$ 18,700,000
Equity Investment Agreement [Member] | Lysogne [Member] | Other Non-current Asset [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount of investment allocated based on closing price of common stock	$ 1,900,000
Equity Investment Agreement [Member] | Lysogne [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory and sales milestones payments to be paid	$ 102,800,000
License Agreement [Member] | Lacerta Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment																			$ 42,000,000
Pompe License [Member] | Lacerta Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment																			8,000,000
Pompe License [Member] | Lacerta Therapeutics, Inc. [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment																			44,000,000
Purchase Agreement [Member] | Lacerta Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment																			$ 30,000,000
Purchase Agreement [Member] | Lacerta Therapeutics, Inc. [Member] | Series A Preferred Stock [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of shares purchased under agreement | shares																			4,500,000
License and Purchase Agreement [Member] | Lacerta Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment																			$ 38,000,000
Options Agreement [Member] | Lacerta Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment																			0
Fair value of options																			$ 0
Warrant Agreement [Member] | Myonexus Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of development milestone | Milestone																2
Research and development expense																$ 85,000,000
Up-front cash payment under agreements			$ 60,000,000													60,000,000
Cash consideration under agreement excluding contingent payments			$ 200,000,000
Development milestone payments evaluation period			2 years
Milestone payment																20,000,000
Milestone accured payment								$ 5,000,000								$ 5,000,000
Warrant Agreement [Member] | Myonexus Therapeutics, Inc. [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional development milestone payments to be paid			$ 45,000,000
Nationwide License Agreement [Member] | Nationwide [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of development milestone | Milestone																0
Research and development expense																$ 0
Up-front cash payment under agreements		$ 1,000,000
Nationwide License Agreement [Member] | Nationwide [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory and sales milestones payments to be paid		$ 29,000,000
Settlement Agreement [Member] | BioMarin Pharmaceutical, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fee to be paid on execution of agreements				20,000,000
License Agreement [Member] | BioMarin and Academisch Ziekenhuis Leiden [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fee to be paid on execution of agreements				$ 15,000,000
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																142,400,000	22,000,000	47,600,000
Collaboration and License Agreement [Member] | Summit [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front cash payment under agreements					$ 40,000,000
One-time option fee					$ 10,000,000
Percentage of budgeted research and development cost allocation					55.00%
Collaboration expense																8,600,000	0	0
Development milestone and up-front payments																$ 0	$ 22,000,000	$ 40,000,000
Collaboration and License Agreement [Member] | Summit [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