Organization and Basis of Presentation - Additional Information (Detail) (USD $)	1 Months Ended		9 Months Ended
	Apr. 30, 2013	Feb. 28, 2013 CreditFacility	Sep. 30, 2013 Segment	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Organization And Nature Of Business [Line Items]
Number of operating segments			1
Losses incurred during the period			$ 534,416,000		$ 431,251,000
Cash equivalents and invested cash			281,400,000
Cash and cash equivalents			273,644,000		187,661,000	37,987,000	39,904,000
Restricted investments included in Cash equivalents and invested cash			7,800,000
Letters of credit amount issued		7,300,000
Certificate of deposit purchased		7,300,000
Certificate of deposit maturity date			2014-04
Restricted investments			7,250,000
Number of letter of credit		2
Upfront and development milestone payments	7,100,000
Construction in progress related to the lease in property and equipment				4,700,000
Tenant improvement from the landlord				3,700,000
Lease Agreements [Member]
Organization And Nature Of Business [Line Items]
Letters of credit amount issued			600,000
Lease agreement, expiration period			7 years
Lease agreement, average base rent due current			2,400,000
Lease agreement, average base rent year one			2,400,000
Lease agreement, average base rent year two			2,400,000
Lease agreement, average base rent year three			2,400,000
Lease agreement, average base rent year four			2,400,000
Lease agreement, average base rent year five			2,400,000
Lease agreement, average base rent year six			2,400,000
Upfront payments recognized under agreement	$ 1,000,000		$ 1,000,000