Organization and Basis of Presentation - Additional Information (Detail) (USD $)	1 Months Ended		6 Months Ended				3 Months Ended	6 Months Ended		1 Months Ended
	Feb. 28, 2013 CreditFacility	Jan. 31, 2013	Jun. 30, 2013 Segment	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2013 License Agreement [Member]	Jun. 30, 2013 License Agreement [Member]	Jun. 30, 2013 Lease Agreements [Member]	Apr. 30, 2013 Intellectual Property Rights [Member] License Agreement [Member]
Organization And Nature Of Business [Line Items]
Number of operating segments			1
Losses incurred during the period			$ 492,386,000	$ 431,251,000
Cash equivalents and invested cash			164,000,000
Cash and cash equivalents			156,185,000	187,661,000	24,491,000	39,904,000
Restricted investments included in Cash equivalents and invested cash			7,800,000
Net proceeds from issue of common stock		2,100,000	37,900,000
Letters of credit amount issued	7,300,000								600,000
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
Lease agreement, expiration period									7 years
Lease agreement, average base rent									2,400,000
Upfront and development milestone payments										7,100,000
Upfront payments recognized under agreement							$ 1,000,000	$ 1,000,000