Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended			112 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012
Cash flows from operating activities:
Net loss	$ (96,132)	$ (63,778)	$ (32,670)	$ (283,724)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	658	268	188	2,574
Stock-based compensation	4,880	2,759	3,637	20,181
Change in fair value of warrants	(6,050)	(7,583)	8,889	(10,771)
Loss on disposal of fixed assets	48			57
Common stock issued in exchange for in-process research and development	18,694	17,457		36,151
(Increase) decrease in:
Collaboration receivable	21	(79)		(58)
Prepaid expenses and other current assets	(5,599)	(889)	(70)	(6,912)
Other noncurrent assets	(230)	(407)	(122)	(1,001)
Deposits	(43)	(4)	(41)	(134)
Increase (decrease) in:
Accounts payable	(218)	696	(758)	1,509
Accrued expenses	5,695	8,283	1,277	16,516
Deferred revenue	(800)	4,333		3,533
Deferred rent	244	109	(24)	439
Net cash used in operating activities	(78,832)	(38,835)	(19,694)	(221,640)
Cash flows from investing activities:
Purchases of property and equipment	(1,559)	(1,156)	(186)	(4,626)
Proceeds from sale of property and equipment				1
Net cash used in investing activities	(1,559)	(1,156)	(186)	(4,625)
Cash flows from financing activities:
Stockholders' capital contribution				500
Proceeds from exercise of stock options	30	980	225	1,373
Payments to employees for repurchase of restricted common stock	(546)	(274)	(1,668)	(2,867)
Proceeds from exercise of warrants	330	12,399	72	13,079
Proceeds from issuance of common stock and warrants, net	49,170	71,207	32,804	270,726
Proceeds from issuance of preferred stock, net				16,760
Net cash provided by financing activities	48,984	84,312	31,433	299,571
Net increase (decrease) in cash and cash equivalents	(31,407)	44,321	11,553	73,306
Cash and cash equivalents, beginning of period	104,713	60,392	48,839
Cash and cash equivalents, end of period	73,306	104,713	60,392	73,306
Supplementary disclosure of cash flow information:
Cash paid for interest
Cash paid for income taxes
Supplementary disclosure of noncash investing and financing activities:
Warrants issued to placement agents and investors				47,276
Exercise of liability-classified warrants to common shares	412	303	49	352
Convertible Preferred Stock
Supplementary disclosure of noncash investing and financing activities:
Preferred stock conversion/exercise of equity-classified warrants to common stock				16,760
Warrant Exercise
Supplementary disclosure of noncash investing and financing activities:
Preferred stock conversion/exercise of equity-classified warrants to common stock	$ 269	$ 9,067	$ 239	$ 9,324