Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended			124 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013
Cash flows from operating activities:
Net Loss	$ (57,107)	$ (96,132)	$ (63,778)	$ (340,831)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	738	658	268	3,313
Stock-based compensation	3,507	4,880	2,759	23,688
Change in fair value of warrants	(1,185)	(6,050)	(7,583)	(11,956)
Loss on disposal of fixed assets	585	48	0	641
Common stock issued in exchange for in-process research and development	0	18,694	17,457	36,151
(Increase) decrease in:
Receivables	(87)	21	(79)	(145)
Prepaid expenses and other current assets	4,964	(5,599)	(889)	(1,948)
Other noncurrent assets	473	(230)	(407)	(528)
Deposits	4	(43)	(4)	(128)
Increase (decrease) in:
Accounts payable	(1,087)	(218)	696	422
Accrued expenses	(10,159)	5,695	8,283	6,357
Deferred revenue	(800)	(800)	4,333	2,733
Deferred rent	625	244	109	1,063
Other noncurrent liabilities	20	0	0	20
Net cash used in operating activities	(59,509)	(78,832)	(38,835)	(281,148)
Cash flows from investing activities:
Purchases of property and equipment	(132)	(1,559)	(1,156)	(4,758)
Proceeds from sale of property and equipment	1	0	0	2
Net cash used in investing activities	(131)	(1,559)	(1,156)	(4,756)
Cash flows from financing activities:
Stockholders' capital contribution	0	0	0	500
Proceeds from exercise of stock options	956	30	980	2,329
Payments to employees for repurchase of restricted common stock	(498)	(546)	(274)	(3,364)
Proceeds from exercise of warrants	200	330	12,399	13,278
Proceeds from issuance of common stock and warrants, net	53,880	49,170	71,207	324,605
Proceeds from issuance of preferred stock, net	0	0	0	16,760
Net cash provided by financing activities	54,538	48,984	84,312	354,108
Net increase (decrease) in cash and cash equivalents	(5,102)	(31,407)	44,321	68,204
Cash and cash equivalents, beginning of period	73,306	104,713	60,392	0
Cash and cash equivalents, end of period	68,204	73,306	104,713	68,204
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	0	0	0	47,276
Exercise of liability-classified warrants to common shares	0	412	303	764
Convertible Preferred Stock
Supplementary disclosure of noncash investing and financing activities:
Preferred stock conversion/exercise of equity-classified warrants to common stock	0	0	0	16,760
Warrants
Supplementary disclosure of noncash investing and financing activities:
Preferred stock conversion/exercise of equity-classified warrants to common stock	$ 196	$ 269	$ 9,067	$ 9,789