CONDENSED CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 97,608	$ 61,349
Marketable securities	101,096	130,180
Accounts receivable	5,872	7,427
Unbilled revenue	2,799	2,909
Prepaid expenses and other current assets	3,308	3,465
Total current assets	210,683	205,330
Property and equipment, net	23,872	25,422
Restricted cash	20,719	20,719
Intangible assets, net	4,324	4,589
Other long-term assets	156	156
Total assets	259,754	256,216
Current liabilities:
Accounts payable	5,494	7,433
Accrued expenses	7,311	10,348
Deferred revenue	9,770	5,490
Other current liabilities	538	518
Total current liabilities	23,113	23,789
Deferred revenue, net of current portion	19,541	25,508
Other long-term liabilities	400	551
Total liabilities	43,054	49,848
Commitments and contingencies (Note 9)
Stockholders' Equity:
Preferred stock, $0.01 par value per share; 5,000 shares authorized at March 31, 2015 and December 31, 2014, 100 shares of Series A Junior Participating Preferred Stock, $0.01 par value per share designated and no shares issued and outstanding
Common stock, $0.0001 par value per share; 100,000 shares authorized at March 31, 2015 and December 31, 2014, 57,458 and 54,486 shares issued and outstanding at March 31, 2015 and December 31, 2014, respectively	5	5
Additional paid-in capital	607,629	575,438
Accumulated other comprehensive income (loss)	2	(16)
Accumulated deficit	(390,936)	(369,059)
Total stockholders' equity	216,700	206,368
Total liabilities and stockholders' equity	$ 259,754	$ 256,216