Commitments and Contingencies - Additional Information (Detail) (USD $)	1 Months Ended		3 Months Ended			9 Months Ended			9 Months Ended	1 Months Ended		9 Months Ended	3 Months Ended	9 Months Ended
	Apr. 30, 2013	Feb. 28, 2013 CreditFacility	Sep. 30, 2014	Jun. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 Corvallis, Oregon [Member]	Jun. 30, 2014 Sublease [Member] sqft	Jan. 31, 2014 Sublease [Member] sqft	Sep. 30, 2014 Lease Agreements [Member] sqft	Sep. 30, 2014 Assignors [Member]	Sep. 30, 2014 Assignors [Member]
Commitments and Contingencies [Line Items]
Letters of credit amount issued		$ 7,300,000										$ 600,000
Total rental space												69,929
Lease expiration date									2020-12	2021-02	2015-07	2021-01
Additional rental space under amendment										10,939	15,077
Rent expense under leases			1,300,000		1,000,000	3,300,000	2,200,000
Notice period to terminate license agreements						6 months
Royalty payments			100,000		100,000	100,000	100,000
Maximum royalty to Charley's Fund						3,400,000
Upfront and development significant milestone payments			0		0	0	0
Obligated to make milestone payments						23,400,000
Receivables under satisfaction of milestones						21,100,000
Upfront payments recognized under agreement	7,100,000		0		0	0	1,000,000						0	300,000
Development and commercial milestone payments														2,700,000
Law affected under lawsuit filling						The complaints allege that the defendants violated the federal securities laws in connection with disclosures related to eteplirsen
Number of days to amend plaintiffs complaint						28 days
Certificate of deposit purchased		7,300,000
Certificate of deposit maturity date						2015-04
Maturity of restricted investments				3,300,000		3,250,000	0
Restricted investments			$ 4,000,000	$ 4,000,000		$ 4,000,000		$ 7,250,000
Number of letter of credit		2