Statements of Operations and Comprehensive Loss (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating expenses:
Research and development	$ 54,961	$ 36,179	$ 11,692
General and administrative	9,469	5,330	3,630
Restructuring			217
Loss from operations	(64,430)	(41,509)	(15,539)
Other (expense) income:
Other (expense) income, net	(2,095)	(236)	824
Interest income	64	27	85
Interest expense	(2,667)	(758)
Net loss	(69,128)	(42,476)	(14,630)
Other comprehensive loss, net of tax:
Unrealized gain (loss) from available-for-sale securities	(5)	8	(18)
Comprehensive loss	(69,133)	(42,468)	(14,648)
(Loss) earnings attributable to common stockholders-basic and diluted (Note 15)	$ (83,120)	$ 253	$ (26,773)
(Loss) earnings per share (Note 15):
Basic (in dollars per share)	$ (98.99)	$ 0.51	$ (83.42)
Diluted (in dollars per share)	$ (98.99)	$ 0.07	$ (83.42)
Weighted average shares:
Basic (in shares)	839,698	499,944	320,924
Diluted (in shares)	839,698	3,454,276	320,924