Condensed Consolidated Statements Of Cash Flows (USD $) In Thousands	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010
Cash flows from operating activities:
Net loss	$ (17,926)	$ (24,705)
Adjustments to reconcile net loss to net cash flows used in operating activities:
Depreciation and amortization	215	213
Accretion of discount on securities	(38)	(174)
Stock-based compensation	7,289	6,705
Changes in operating assets and liabilities:
Receivable from collaboration partners	6,425	(6,543)
Prepaid expenses and other current assets	1,276	1,181
Other non-current assets	(1,874)	(414)
Accounts payable	3,056	2,078
Accrued expenses	2,802	4,282
Other current liabilities	(4,734)	(647)
Deferred revenue	(27,508)	(31,526)
Other non-current liabilities	(142)	(109)
Net cash used in operating activities	(31,159)	(49,659)
Cash flows from investing activities:
Purchase of short-term investments		(69,962)
Maturities of short-term investments	100,000	86,000
Purchase of property and equipment	(148)	(102)
Net cash provided by investing activities	99,852	15,936
Cash flows from financing activities:
Stock option exercises	2,917	3,061
Net proceeds from stockholder securities law settlement	2,959
Net cash provided by financing activities	5,876	3,061
Net increase (decrease) in cash and cash equivalents	74,569	(30,662)
Cash and cash equivalents at beginning of period	107,717	57,463
Cash and cash equivalents at end of period	$ 182,286	$ 26,801