Statements of Operations (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended		81 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013
Costs and expenses:
Research and development	$ 11,101	$ 1,595	$ 17,836	$ 4,794	$ 92,727
General and administrative	4,166	2,259	9,145	5,341	36,494
Total costs and expenses	15,267	3,854	26,981	10,135	129,221
Loss from operations	(15,267)	(3,854)	(26,981)	(10,135)	(129,221)
Interest expense		(230)	(1,454)	(256)	(1,964)
Interest and other income					481
Gain (loss) on extinguishment of debt	105		(1,091)		(1,091)
Other loss	(970)	(69)	(1,231)	(340)	(1,602)
Change in fair value related to investor rights liability					683
Net loss before income tax benefit	(16,132)	(4,153)	(30,757)	(10,731)	(132,714)
Income tax benefit					1,327
Net loss	(16,132)	(4,153)	(30,757)	(10,731)	(131,387)
Add: accretion of preferred stock dividends	(2,292)	(1,775)	(5,891)	(5,288)	(33,046)
Net loss attributable to common stockholders	$ (18,424)	$ (5,928)	$ (36,648)	$ (16,019)	$ (164,433)
Net loss attributable to common stockholders per share -basic and diluted (in dollars per share)	$ (10.26)	$ (4.07)	$ (23.21)	$ (11.07)
Weighted average common shares outstanding:
Basic and diluted (in shares)	1,795	1,455	1,579	1,447