Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Cash flows from operating activities:
Net loss	$ (15,287)	$ (31,491)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	217	396
Stock-based compensation	2,435	1,673
Change in fair value of warrants	(5,682)	(10,385)
(Increase) decrease in:
Receivables	113	27
Prepaid expenses and other current assets	957	2,845
Other noncurrent assets	15	269
Deposits		4
Increase (decrease) in:
Accounts payable	25	(344)
Accrued expenses	1,416	2,453
Deferred revenue	(400)	(400)
Deferred rent	(98)	(11)
Net cash used in operating activities	(16,289)	(34,964)
Cash flows from investing activities:
Purchases of property and equipment	(99)	(115)
Net cash used in investing activities	(99)	(115)
Cash flows from financing activities:
Proceeds from exercise of stock options	1,317	854
Payments to employees for repurchase of common stock	(132)	(250)
Proceeds from exercise of warrants		101
Net cash provided by financing activities	1,185	705
Net increase (decrease) in cash and cash equivalents	(15,203)	(34,374)
Cash and cash equivalents, beginning of period	68,204	73,306
Cash and cash equivalents, end of period	53,001	38,932
Supplementary disclosure of cash flow information:
Cash paid for interest	0	0
Cash paid for income taxes	0	0
Supplementary disclosure of noncash investing and financing activities:
Exercise of equity-classified warrants to common shares	$ 62	$ 72