STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net loss	$ (31,781)	$ (57,107)	$ (96,132)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	462	738	658
Stock-based compensation	4,743	3,507	4,880
Change in fair value of warrants	(11,723)	(1,185)	(6,050)
Loss on disposal of fixed assets		585	48
Common stock issued in exchange for in-process research and development			18,694
(Increase) decrease in:
Receivables		(87)	21
Prepaid expenses and other current assets	809	4,964	(5,599)
Other noncurrent assets	37	473	(230)
Deposits		4	(43)
Increase (decrease) in:
Accounts payable	1,582	(1,087)	(218)
Accrued expenses	827	(10,159)	5,695
Deferred revenue	(1,373)	(800)	(800)
Deferred rent	(213)	625	244
Other noncurrent liabilities	(20)	20
Net cash used in operating activities	(36,650)	(59,509)	(78,832)
Cash flows from investing activities:
Purchases of property and equipment	(193)	(132)	(1,559)
Proceeds from sale of property and equipment		1
Net cash used in investing activities	(193)	(131)	(1,559)
Cash flows from financing activities:
Proceeds from exercise of stock options	1,386	956	30
Payments to employees for repurchase of restricted common stock	(544)	(498)	(546)
Proceeds from exercise of warrants	10,600	200	330
Proceeds from issuance of common stock and warrants, net		53,880	49,170
Net cash provided by financing activities	11,442	54,538	48,984
Net decrease in cash and cash equivalents	(25,401)	(5,102)	(31,407)
Cash and cash equivalents, beginning of period	68,204	73,306	104,713
Cash and cash equivalents, end of period	42,803	68,204	73,306
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:
Exercise of liability-classified warrants to common shares	54		412
Warrants
Supplementary disclosure of noncash investing and financing activities:
Exercise of equity-classified warrants to common shares	$ 692	$ 196	$ 269