Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Cash flows from operating activities:
Net loss	$ (90,192)	$ (62,790)	$ (47,380)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,030	3,322	3,354
Depreciation and amortization	516	738	1,464
Changes in operating assets and liabilities:
Collaboration receivable			116
Prepaid expenses and other current assets	21	(225)	(130)
Other assets	(951)	173	(265)
Accounts payable	928	2,194	1,542
Accrued contract research costs	5,646	1,920	330
Other accrued liabilities	591	533	400
Deferred collaboration revenue			(6,731)
Net cash used in operating activities	(77,411)	(54,135)	(47,300)
Cash flows from investing activities:
Purchases of marketable securities	(114,151)	(50,033)	(50,726)
Maturities of marketable securities	90,267	40,595	60,745
Purchases of property and equipment	(769)	(505)	(690)
Net cash (used in) provided by investing activities	(24,653)	(9,943)	9,329
Cash flows from financing activities:
Proceeds from issuances of common stock, excluding to related parties, and exercise of common stock options, net of transaction costs	38,735	66,248	27,581
Proceeds from the sale of common stock to related parties	19,437	53,545	7,734
Purchase and retirement of common stock from an officer		(32)
Proceeds from term loans	13,500		2,000
Payment of term loans	(2,617)	(4,234)	(378)
Payment of capital lease obligations	(13)	(12)	(201)
Net cash provided by financing activities	69,042	115,515	36,736
Net increase (decrease) in cash and cash equivalents	(33,022)	51,437	(1,235)
Cash and cash equivalents at beginning of period	81,512	30,075	31,310
Cash and cash equivalents at end of period	48,490	81,512	30,075
Supplemental disclosure of cash flow information:
Cash paid for interest	2,512	1,696	1,911
Assets acquired under capital lease	$ 126