Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Cash flows from operating activities:
Net loss	$ (44,667)	$ (36,637)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,359	2,506
Depreciation and amortization	623	1,173
Changes in operating assets and liabilities:
Collaboration receivable		116
Prepaid expenses and other current assets	(242)	(931)
Other assets	165	(164)
Accounts payable	223	997
Accrued contract research costs	878	499
Other accrued liabilities	(951)	(945)
Deferred collaboration revenue		(3,428)
Net cash used in operating activities	(41,612)	(36,814)
Cash flows from investing activities:
Purchases of marketable securities	(45,887)	(46,994)
Maturities of marketable securities	29,845	36,644
Purchases of property and equipment	(421)	(308)
Net cash used in investing activities	(16,463)	(10,658)
Cash flows from financing activities:
Proceeds from the issuance of common stock, excluding to related parties, net of transaction costs	28,960	27,494
Proceeds from the issuance of common stock to related parties	30,759	7,734
Proceeds from term loans		2,000
Payment of term loans	(2,268)	(233)
Payment of capital lease obligations	(9)	(190)
Net cash provided by financing activities	57,442	36,805
Net decrease in cash and cash equivalents	(633)	(10,667)
Cash and cash equivalents at beginning of period	30,075	31,310
Cash and cash equivalents at end of period	29,442	20,643
Supplemental disclosure of cash flow information:
Cash paid for interest	$ 1,335	$ 1,484