Significant Accounting Policies and Recent Accounting Pronouncements - Additional Information (Detail) $ in Thousands	Sep. 30, 2016 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Sales milestone recorded as an in-license right	$ 1,000
EXONDYS 51 [Member]
Summary Of Significant Accounting Policies [Line Items]
Sales milestone recorded as an in-license right	$ 1,000

Gross carrying amount as of the balance sheet date of certain rights acquired to exercise a certain privilege or pursue a particular business or occupation and which is deemed to have a finite economic life.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

Name:	us-gaap_FiniteLivedLicenseAgreementsGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant