Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating activities
Net loss	$ (41,374)	$ (59,664)	$ (6,434)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	561	343	54
Stock based compensation expense	2,981	1,599	366
Gain on sale of asset, net	(5)	(1)	0
Gain on sale of short-term investments, net	(833)	0	0
Impairment loss	0	25,990	0
Amortization of debt discount and debt issuance costs	236	0	0
Accrual of the end of term charge on the debt	44	0	0
Changes in operating assets and liabilities:
Accounts receivable	757	(286)	19
Income tax receivable	0	0	2,023
Prepaid expenses and other assets	(180)	(214)	(78)
Accounts payable	4,726	884	(2,750)
Accrued expenses and deferred rent	922	667	(1,127)
Accrued lease expenses	(259)	1,201	0
Accrued compensation	1,412	(322)	201
Deferred revenue	0	(402)	4
Net cash used in operating activities	(31,012)	(30,205)	(7,722)
Investing activities
Cash consideration for merger, net of cash acquired	0	0	(6,733)
Purchase of fixed assets	(290)	(979)	0
Proceeds from sale of asset	5	0	0
Sales of short-term investments	81,464	36,500	115,153
Purchases of short-term investments	(19,657)	(1,161)	(102,462)
Net cash provided by investing activities	61,522	34,360	5,958
Financing activities
Payments on capital lease obligations	(120)	(82)	(6)
Proceeds from issuance of debt	20,000	0	0
Repayment of convertible notes	0	0	(231)
Proceeds from issuance of common stock, net of issuance costs	25,658	0	0
Proceeds from exercise of stock options	214	32	0
Payment of debt issuance costs	(328)	0	0
Other	0	0	4
Net cash provided by (used in) financing activities	45,424	(50)	(233)
Increase (decrease) in cash and cash equivalents	75,934	4,105	(1,997)
Cash and cash equivalents at beginning of period	14,848	10,743	12,740
Cash and cash equivalents at end of period	90,782	14,848	10,743
Supplemental disclosures of cash flow information:
Cash paid for interest	398	10	0
Cash received for taxes, net	0	0	(1,884)
Supplemental disclosures of non-cash investing and financing activities:
Capital lease obligations incurred	0	198	0
Unrealized (gain) loss on investments	(450)	(543)	548
Accretion of beneficial conversion charge	0	(9,175)	0
Fair value of warrant granted in connection with debt financing	790	0	0
Conversion of Series B preferred stock into common stock	0	918	0
Issuance of common stock upon merger	0	0	18,419
Issuance of preferred stock upon merger	$ 0	$ 0	$ 56,026