Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Cash flows from operating activities:
Net loss	$ (66,030)	$ (44,667)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	4,490	2,359
Depreciation and amortization	373	623
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(572)	(242)
Other assets	25	165
Accounts payable	32	223
Accrued contract research costs	3,591	878
Other accrued liabilities	(630)	(951)
Net cash used in operating activities	(58,721)	(41,612)
Cash flows from investing activities:
Purchases of marketable securities	(71,183)	(45,887)
Maturities of marketable securities	62,367	29,845
Purchases of property and equipment	(687)	(421)
Net cash used in investing activities	(9,503)	(16,463)
Cash flows from financing activities:
Proceeds from issuance of common stock, excluding to related parties, and exercise of common stock options, net of transaction costs	1,082	28,960
Proceeds from the sale of common stock to related parties		30,759
Proceeds from term loans	13,500
Payment of term loans	(2,383)	(2,268)
Payment of capital lease obligations	(10)	(9)
Net cash provided by financing activities	12,189	57,442
Net decrease in cash and cash equivalents	(56,035)	(633)
Cash and cash equivalents at beginning of period	81,512	30,075
Cash and cash equivalents at end of period	25,477	29,442
Supplemental disclosure of cash flow information:
Cash paid for interest	$ 1,918	$ 1,335