Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
CASH FLOWS USED IN OPERATING ACTIVITIES
Net loss	$ (69,128)	$ (42,476)	$ (14,630)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	44	40	70
Gain on fixed asset disposal			(100)
Amortization of premium (accretion of discount) on short-term investments, net	101	21	303
Stock-based compensation expense	1,795	304	134
Research and development expense settled in stock	15,067	10,296
Change in fair value of other current assets, warrant liability and other liability	2,069	264
Non-cash interest	449	165
Milestone payment settled with stock		1,410
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	4,623	(6,463)	(133)
Other long-term assets	35	25	(27)
Accounts payable	237	(301)	(94)
Accrued expenses and other current liabilities	1,550	819	1,491
Net cash used in operating activities	(43,158)	(35,896)	(12,986)
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES
Purchases of property and equipment	(40)	(176)	(14)
Proceeds from sale of equipment			149
Purchases of marketable securities	(18,989)	(32,479)	(24,120)
Sales and maturities of marketable securities	46,464	8,855	39,655
Net cash (used in) provided by investing activities	27,435	(23,800)	15,670
CASH FLOWS PROVIDED BY FINANCING ACTIVITIES
Proceeds from the sale of common stock			2
Proceeds from exercise of stock options	279	204
Net proceeds from the issuance of preferred stock		62,116
Proceeds from note payable	12,500	12,500
Discount on note payable		(366)
Deferred financing costs	(31)	(56)
Payments on note payable	(3,500)	(156)
Net cash provided by financing activities	9,248	74,242	2
Net (decrease) increase in cash and cash equivalents	(6,475)	14,546	2,686
Cash and cash equivalents at beginning of year	25,128	10,582	7,896
Cash and cash equivalents at end of year	18,653	25,128	10,582
SUPPLEMENTAL DISCLOSURES
Cash paid for interest	1,801	358
NON-CASH FINANCING ACTIVITIES
Accretion of preferred stock issuance costs			173
Accretion of dividends on preferred stock	13,992	10,933	10,651
Accretion of preferred stock investor rights/obligations			1,318
Fair value of preferred stock issued in the recapitalization		85,879
Fair value of warrants issued	$ 379	$ 463