CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Cash flows from operating activities:
Net loss	$ (13,480)	$ (12,561)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	409	451
Amortization of premium/(discount) on marketable securities	633	1,180
Amortization of deferred gain on sale leaseback	(278)	(278)
Non-cash stock compensation	1,880	2,312
Loss (gain) on auction rate securities	(77)	70
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(356)	(616)
Other long-term assets	355	128
Accounts payable and accrued expenses	(616)	(1,977)
Deferred revenue	(5,464)	(7,099)
Net cash used in operating activities	(16,994)	(18,390)
Cash flows from investing activities:
Purchases of marketable securities	(14,100)	(9,063)
Proceeds from sale or maturity of marketable securities	26,672	26,176
Net cash provided by investing activities	12,572	17,113
Cash flows from financing activities:
Proceeds from stock option exercises and employee stock plan purchases	57	213
Net cash provided by financing activities	57	213
Net decrease in cash and cash equivalents	(4,365)	(1,064)
Cash and cash equivalents, beginning of period	15,579	14,327
Cash and cash equivalents, end of period	$ 11,214	$ 13,263