CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Cash flows from operating activities:
Net loss	$ (19,879)	$ (18,644)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	532	658
Amortization of premium on marketable securities	893	1,682
Amortization of deferred gain on sale leaseback	(416)	(416)
Non-cash stock compensation	2,643	3,201
Loss (gain) on auction rate securities	(75)	66
Impairment of property and equipment	280
Changes in operating assets and liabilities:
Prepaid expenses and other assets	558	142
Accounts payable and accrued expenses	(1,275)	(2,073)
Deferred revenue	(8,228)	(10,609)
Net cash used in operating activities	(24,967)	(25,993)
Cash flows from investing activities:
Purchases of marketable securities	(27,504)	(29,275)
Proceeds from sale or maturity of marketable securities	52,934	57,777
Net cash provided by investing activities	25,430	28,502
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 increase in cash and cash equivalents	520	2,722
Cash and cash equivalents, beginning of period	15,579	14,327
Cash and cash equivalents, end of period	$ 16,099	$ 17,049