CONDENSED CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 43,288	$ 52,990
Marketable securities	280,916	287,613
Accounts receivable	5,396	10,811
Unbilled revenue	1,232	800
Prepaid expenses and other current assets	3,639	4,953
Total current assets	334,471	357,167
Property and equipment, net	21,400	22,380
Restricted cash	19,971	19,971
Intangible assets, net	6,446	6,711
Other long-term assets	400	400
Total assets	382,688	406,629
Current liabilities:
Accounts payable	3,643	3,580
Accrued expenses	7,299	9,641
Deferred revenue	4,576	4,426
Other current liabilities	372	514
Total current liabilities	15,890	18,161
Deferred revenue, net of current portion	26,096	27,269
Other long-term liabilities	657	712
Total liabilities	42,643	46,142
Commitments and contingencies (Note 9)
Stockholders' Equity:
Common stock, $0.0001 par value per share; 100,000 shares authorized at March 31, 2013 and December 31, 2012, 51,925 and 51,709 shares issued and outstanding at March 31, 2013 and December 31, 2012, respectively	5	5
Additional paid-in capital	526,161	522,422
Accumulated other comprehensive income	46	111
Accumulated deficit	(186,167)	(162,051)
Total stockholders' equity	340,045	360,487
Total liabilities and stockholders' equity	382,688	406,629
Preferred stock
Stockholders' Equity:
Preferred stock
Series A Junior Participating Preferred Stock
Stockholders' Equity:
Preferred stock