Intangible Assets - Additional Information (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jul. 31, 2017
Finite Lived Intangible Assets [Line Items]
Amortization of in-licensed rights	$ 200,000	$ 216,000	$ 217,000	$ 216,000	$ 216,000	$ 216,000	$ 217,000	$ 216,000	$ 849,000	$ 865,000	$ 1,053,000
Patents [Member]
Finite Lived Intangible Assets [Line Items]
Amortization of in-licensed rights									400,000	700,000	600,000
Patents [Member] | Research and Development [Member]
Finite Lived Intangible Assets [Line Items]
Book value of patent abandoned									200,000	100,000	600,000
Impairment loss on patent assets									0	3,800,000	0
Bio Marin [Member]
Finite Lived Intangible Assets [Line Items]
Amortization of in-licensed rights									1,700,000	$ 2,100,000	1,700,000
Maximum [Member]
Finite Lived Intangible Assets [Line Items]
Amortization of in-licensed rights											$ 1,100,000
EXONDYS 51 [Member]
Finite Lived Intangible Assets [Line Items]
In-lincensed Rights									1,000,000.0
EXONDYS 51 [Member] | Maximum [Member] | Bio Marin [Member]
Finite Lived Intangible Assets [Line Items]
Sales milestone payment recorded as an in-license right												$ 6,600,000
VYONDYS 53 [Member]
Finite Lived Intangible Assets [Line Items]
In-lincensed Rights									$ 500,000