Condensed Consolidated Statements of Cash Flows (USD $) In Thousands	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010
Cash flows from operating activities:
Net loss	$ (36,637)	$ (28,644)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,506	3,185
Depreciation and amortization	1,173	1,470
Changes in operating assets and liabilities:
Collaboration receivable	116	(189)
Prepaid expenses and other current assets	(931)	(74)
Other assets	(164)	(103)
Accounts payable	997	(1,564)
Accrued contract research costs	499	627
Other accrued liabilities	(945)	(1,191)
Deferred collaboration revenue	(3,428)	(3,504)
Net cash used in operating activities	(36,814)	(29,987)
Cash flows from investing activities:
Purchases of marketable securities	(46,994)	(19,213)
Maturities of marketable securities	36,644
Purchases of property and equipment	(308)	(109)
Net cash used in investing activities	(10,658)	(19,322)
Cash flows from financing activities:
Proceeds from issuance of common stock and exercise of common stock options, net of transaction costs	35,228	26,807
Proceeds from term loans	2,000	15,000
Payment of term loans	(233)
Payment of capital lease obligations	(190)	(1,747)
Net cash provided by financing activities	36,805	40,060
Net decrease in cash and cash equivalents	(10,667)	(9,249)
Cash and cash equivalents at beginning of period	31,310	44,155
Cash and cash equivalents at end of period	20,643	34,906
Supplemental disclosure of cash flow information:
Cash paid for interest	$ 1,464	$ 120