Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Additional Information (Detail)	3 Months Ended					12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment							1
Restricted cash and investments	$ 784,000				$ 784,000		$ 784,000	$ 784,000
Amortization of in-licensed rights	216,000	$ 780,000	$ 29,000	$ 28,000	29,000		1,053,000	29,000
Debt issuance costs							15,154,000		$ 399,000
Prepaid maintenance services	1,711,000				109,000		1,711,000	109,000
Prepaid clinical and preclinical expenses	5,175,000				1,225,000		5,175,000	1,225,000
Prepaid commercial expenses	1,589,000				54,000		1,589,000	54,000
Accrued interest expenses	1,291,000				100,000		1,291,000	100,000
Product revenue related reserves	7,302,000				305,000		7,302,000	305,000
Accrued property and equipment	2,525,000				1,186,000		2,525,000	1,186,000
Intangible asset	14,355,000				8,076,000		14,355,000	$ 8,076,000
Deferred revenue	3,300,000						$ 3,300,000
U.S. federal corporate tax rate							34.00%	34.00%	34.00%
Transition tax							$ 0
Rent expense and occupancy costs							4,400,000	$ 5,600,000	$ 5,200,000
ASU No. 2016-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash included in cash and cash equivalents	100,000						$ 100,000
Employee Stock Purchase Plan [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of closing price of common stock							85.00%
Percentage of discount recognized on graded-vesting based on stock-based compensation expense over the purchase period							15.00%
Patents [Member]
Summary Of Significant Accounting Policies [Line Items]
Amortization of in-licensed rights							$ 600,000	600,000	500,000
Initial term of patents							20 years
Patents [Member] | Research and Development Expenses [Member]
Summary Of Significant Accounting Policies [Line Items]
Book value of patent abandoned							$ 600,000	300,000	200,000
EXONDYS 51 [Member]
Summary Of Significant Accounting Policies [Line Items]
Amortization of in-licensed rights							1,100,000
Sales milestone recorded as an in-license right	6,600,000				1,000,000		6,600,000	1,000,000
EXONDYS 51 [Member] | University of Western Australia [Member]
Summary Of Significant Accounting Policies [Line Items]
Sales milestone recorded as an in-license right	1,000,000						$ 1,000,000
Product Revenues [Member] | Customer One [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage							47.00%
Product Revenues [Member] | Customer Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage							34.00%
Product Revenues [Member] | Customer Three [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage							19.00%
Accounts Receivable [Member] | Customer One [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage							56.00%
Accounts Receivable [Member] | Customer Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage							27.00%
Accounts Receivable [Member] | Customer Three [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage							16.00%
U.S. [Member] | EXONDYS 51 [Member]
Summary Of Significant Accounting Policies [Line Items]
Intangible asset	$ 6,600,000						$ 6,600,000
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term							60 days
U.S. federal corporate tax rate							35.00%
Maximum [Member] | EXONDYS 51 [Member]
Summary Of Significant Accounting Policies [Line Items]
Amortization of in-licensed rights								100,000
Maximum [Member] | Outside of U.S. [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term							120 days
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term							30 days
Minimum [Member] | Outside of U.S. [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term							60 days
Scenario, Previously Reported [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash and investments					800,000			800,000
Restatement Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Debt issuance costs									$ 400,000
Prepaid maintenance services					100,000			100,000
Prepaid clinical and preclinical expenses					1,200,000			1,200,000
Accrued interest expenses					100,000			100,000
Product revenue related reserves					300,000			300,000
Accrued property and equipment					1,200,000			1,200,000
Restatement Adjustment [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Amortization of in-licensed rights								100,000
Prepaid commercial expenses					$ 100,000			$ 100,000
Scenario, Forecast [Member]
Summary Of Significant Accounting Policies [Line Items]
U.S. federal corporate tax rate						21.00%