Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended		109 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012
Cash flows from operating activities:
Net loss	$ (65,907)	$ (50,534)	$ (253,499)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	466	172	2,382
Stock-based compensation	3,482	1,900	18,783
Change in fair value of warrants	2,516	(4,423)	(2,205)
Loss on disposal of fixed assets	48	0	57
Common stock issued in exchange for in-process research and development	0	17,457	17,457
(Increase) decrease in:
Collaboration receivable	66	(79)	(13)
Prepaid expenses and other current assets	(5,922)	(981)	(7,235)
Other noncurrent assets	(47)	(518)	(818)
Deposits	(42)	(4)	(133)
Increase (decrease) in:
Accounts payable	2,521	1,129	4,248
Accrued expenses	5,861	6,251	16,682
Deferred revenue	(600)	4,533	3,733
Deferred rent	226	12	421
Net cash used in operating activities	(57,332)	(25,085)	(200,140)
Cash flows from investing activities:
Purchases of property and equipment	(1,482)	(806)	(4,549)
Proceeds from sale of property and equipment	0	0	1
Net cash used in investing activities	(1,482)	(806)	(4,548)
Cash flows from financing activities:
Stockholders' capital contribution	0	0	500
Proceeds from exercise of stock options	30	904	1,373
Payments to employees for repurchase of common stock	(96)	(78)	(2,417)
Proceeds from exercise of warrants	330	12,399	13,079
Proceeds from issuance of common stock and warrants, net	49,170	71,207	270,726
Proceeds from issuance of preferred stock, net	0	0	16,760
Net cash provided by financing activities	49,434	84,432	300,021
Net increase (decrease) in cash and cash equivalents	(9,380)	58,541	95,333
Cash and cash equivalents, beginning of period	104,713	60,392	0
Cash and cash equivalents, end of period	95,333	118,933	95,333
Supplementary disclosure of cash flow information:
Cash paid for interest	0	0	0
Cash paid for income taxes	0	0	0
Supplementary disclosure of noncash investing and financing activities:
Warrants issued to placement agents and investors	0	0	47,276
Exercise of liability-classified warrants to common shares	412	303	764
Convertible Preferred Stock [Member]
Supplementary disclosure of noncash investing and financing activities:
Preferred stock conversion/exercise of equity-classified warrants to common stock			16,760
Warrant Exercise [Member]
Supplementary disclosure of noncash investing and financing activities:
Preferred stock conversion/exercise of equity-classified warrants to common stock	$ 269	$ 9,067	$ 9,593