Summary of Product Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation Of Revenue [Line Items]
Products, net	$ 166,911	$ 121,429	$ 433,676	$ 333,221
EXONDYS 51 [Member]
Disaggregation Of Revenue [Line Items]
Products, net	115,598	111,802	335,244	314,332
VYONDYS 53 [Member]
Disaggregation Of Revenue [Line Items]
Products, net	24,658	9,627	64,648	18,889
AMONDYS 45 [Member]
Disaggregation Of Revenue [Line Items]
Products, net	$ 26,655	$ 0	$ 33,784	$ 0