Summary of Product Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation Of Revenue [Line Items]
Products, net	$ 141,839	$ 111,344	$ 266,765	$ 211,792
EXONDYS 51 [Member]
Disaggregation Of Revenue [Line Items]
Products, net	112,461	104,347	219,646	202,530
VYONDYS 53 [Member]
Disaggregation Of Revenue [Line Items]
Products, net	22,442	$ 6,997	39,990	$ 9,262
AMONDYS 45 [Member]
Disaggregation Of Revenue [Line Items]
Products, net	$ 6,936		$ 7,129