LICENSE AND COLLABORATION AGREEMENTS - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Dec. 31, 2025
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Product revenues, net		$ 323,763	$ 370,043	$ 982,967	$ 1,494,689
Collaboration and other		57,588	29,313	530,438	260,614
Loss on strategic investments		$ 523	(36,739)	$ 2,683	17,268
Common stock, shares		198,000,000		198,000,000		198,000,000
ELEVIDYS [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Product revenues, net		$ 83,450	131,498	$ 283,540	788,334
Roche Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and other				325,000	112,000
Arrowhead Pharmaceuticals Inc [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development expense		5,700	7,000	20,500	15,800
Collaborative Arrangement | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development Milestone and Upfront Fee Recognized as Research and Development Expense				12,000,000
Collaborative Arrangement | Roche Holding A.G. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and other				365,000	175,500
Deferred revenue		188,800		188,800		$ 527,300
Deferred Revenue, Current		104,900		104,900		443,400
Research and development expense		7,100	$ 12,200	22,500	$ 66,400
Collaboration Receivable		41,500		41,500		104,100
Collaborative Arrangement | Roche Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and other				40,000
Collaborative Arrangement | NationWide Children Hospital [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone Payment Received	$ 63,500
Collaborative Arrangement | Arrowhead Pharmaceuticals Inc [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development expense				583,600
Equity Investment Agreement [Member] | Arrowhead Pharmaceuticals Inc [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Investment in license agreement		241,400		241,400
Licensing And Collaboration Agreement [Member] | Arrowhead Pharmaceuticals Inc [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up front payment for license and collaboration agreement				500,000
Investment in license agreement		$ 325,000		$ 325,000
Term of installment fee				five years
License agreement fee				$ 250,000
Annual Installment Fee				$ 50,000
Upfront Fee To Be Paid On Certain Enrollment Achievement						$ 300,000
Common stock, shares		11,900,000		11,900,000
Development Milestone and Upfront Fee Recognized as Research and Development Expense				$ 10,300,000
Licensing And Collaboration Agreement [Member] | Second Arrowhead DM One Milestone [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash		$ 200,000		$ 200,000