Commitments and Contingencies - Additional Information (Detail) (USD $)	1 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended	6 Months Ended	1 Months Ended			6 Months Ended		6 Months Ended
	Nov. 30, 2013 sqft	Apr. 30, 2013	Feb. 28, 2013 CreditFacility	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Assignors [Member]	Jun. 30, 2014 Assignors [Member]	Jun. 30, 2014 Sublease [Member] sqft	Jan. 31, 2014 Sublease [Member] sqft	Jun. 30, 2014 Sublease [Member] Maximum [Member]	Jun. 30, 2014 Lease Agreements [Member]	Jun. 30, 2013 Lease Agreements [Member]	Jun. 30, 2014 Corvallis, Oregon [Member]
Commitments and Contingencies [Line Items]
Letters of credit amount issued			$ 7,300,000												$ 600,000
Lease agreement, expiration period														7 years
Lease agreement, average base rent due current				4,026,000		4,026,000								2,400,000
Lease agreement, average annual base rent year two				4,169,000		4,169,000								2,400,000
Lease agreement, average annual base rent year three				4,271,000		4,271,000								2,400,000
Lease agreement, average annual base rent year four				4,374,000		4,374,000								2,400,000
Lease agreement, average base rent year five														2,400,000
Lease agreement, average base rent year six														2,400,000
Lease agreement, average base rent year seven														2,400,000
Additional rental space under amendment	15,077										10,939	15,077
Total rental space	61,453
Additional annual base rent	500,000
Percentage of annual increase in lease	2.50%															3.00%
Lease agreement period												18 months	7 years
Total subleases base rent											300,000	700,000
Annual base rent																900,000
Rent expense under leases				900,000	700,000	2,000,000	1,300,000
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	1,000,000	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						2014-07
Maturity of restricted investments				3,300,000		3,250,000	0
Restricted investments				$ 4,000,000		$ 4,000,000		$ 7,250,000
Number of letter of credit			2