Consolidated Statements of Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	4 Months Ended	12 Months Ended			88 Months Ended
	Dec. 31, 2012	Aug. 31, 2012	Aug. 31, 2011	Aug. 31, 2010	Dec. 31, 2012
Consolidated Statements of Comprehensive Loss [Abstract]
Revenues	$ 0	$ 0	$ 0	$ 0	$ 0
Operating expenses:
General and administrative	8,971	14,723	6,178	3,720	40,547
Research and development	8,963	21,443	14,788	9,334	69,644
Total operating expenses	17,934	36,166	20,966	13,054	110,191
Loss from operations	(17,934)	(36,166)	(20,966)	(13,054)	(110,191)
Interest income	160	340	45	26	873
Interest expense	(83)	(3)	(2)	(5)	(202)
Foreign currency transaction gain	113	145	29	0	287
Gain (loss) on short-term investments	(64)	213	0	0	148
Adjustment to the fair value of common stock warrants	(1,484)	(3,173)	(16,301)	(5,895)	(26,853)
Net loss	(19,292)	(38,644)	(37,195)	(18,928)	(135,938)
Other comprehensive gain (loss)
Foreign currency translation adjustment	(65)	(52)	10	(8)	(115)
Comprehensive loss	$ (19,357)	$ (38,696)	$ (37,185)	$ (18,936)	$ (136,053)
Net loss per share:
Basic and diluted (in dollars per share)	$ (0.37)	$ (0.80)	$ (1.15)	$ (0.85)
Weighted average shares outstanding used to compute:
Basic and diluted (in shares)	51,737	48,085	32,327	22,227