CONDENSED CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 37,131	$ 52,990
Marketable securities	238,766	287,613
Accounts receivable	6,402	10,811
Unbilled revenue	5,345	800
Prepaid expenses and other current assets	5,378	4,953
Total current assets	293,022	357,167
Property and equipment, net	24,395	22,380
Restricted cash	20,719	19,971
Intangible assets, net	5,915	6,711
Other long-term assets	156	400
Total assets	344,207	406,629
Current liabilities:
Accounts payable	6,194	3,580
Accrued expenses	11,774	9,641
Deferred revenue	4,438	4,426
Other current liabilities	479	514
Total current liabilities	22,885	18,161
Deferred revenue, net of current portion	24,260	27,269
Other long-term liabilities	1,074	712
Total liabilities	48,219	46,142
Commitments and contingencies (Note 8)
Stockholders' Equity:
Common stock, $0.0001 par value per share; 100,000 shares authorized at September 30, 2013 and December 31, 2012, 52,289 and 51,709 shares issued and outstanding at September 30, 2013 and December 31, 2012, respectively	5	5
Additional paid-in capital	536,261	522,422
Accumulated other comprehensive income	120	111
Accumulated deficit	(240,398)	(162,051)
Total stockholders' equity	295,988	360,487
Total liabilities and stockholders' equity	344,207	406,629
Preferred stock
Stockholders' Equity:
Preferred stock
Series A Junior Participating Preferred Stock
Stockholders' Equity:
Preferred stock