Summary of Product Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation Of Revenue [Line Items]
Products, net	$ 124,926	$ 100,448
EXONDYS 51 [Member]
Disaggregation Of Revenue [Line Items]
Products, net	107,185	98,183
VYONDYS 53 [Member]
Disaggregation Of Revenue [Line Items]
Products, net	17,548	$ 2,265
AMONDYS 45 [Member]
Disaggregation Of Revenue [Line Items]
Products, net	$ 193