Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Feb. 21, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment	1
Accrued facility-related costs		$ 100,000
Capitalized inventory	$ 36,282,000	19,018,000
Net operating loss carryforwards		157,800,000
Deferred revenue	3,300,000
Rent expense and occupancy costs	5,600,000	5,200,000	$ 4,400,000
Increase in cash and cash equivalents	42,116,000	6,753,000	(183,414,000)
ASU 2016-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Increase in cash and cash equivalents	$ 11,500,000
Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Up-front payment to be received				$ 125,000
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]
Initial term of patents	20 years
Patent amortization expense	$ 600,000	500,000	500,000
Patents [Member] | Research and Development Expenses [Member]
Summary Of Significant Accounting Policies [Line Items]
Book value of patent abandoned	300,000	200,000	100,000
EXONDYS 51 [Member]
Summary Of Significant Accounting Policies [Line Items]
Sales milestone recorded as an in-license right	$ 1,000,000
Product Revenues [Member] | Customer One [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage	80.00%
Product Revenues [Member] | Customer Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage	16.00%
Accounts Receivable [Member] | Customer One [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage	75.00%
Accounts Receivable [Member] | Customer Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage	21.00%
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term	30 days
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term	45 days
Maximum [Member] | EXONDYS 51 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cost of sales related to amortization of in-license right	$ 100,000
Reclassification [Member]
Summary Of Significant Accounting Policies [Line Items]
Proceeds from exercise of options and warrants and the sale of common stock, net of offering costs		$ 130,800,000	$ 104,200,000