Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Cash flows from operating activities:
Net loss	$ (45,843)	$ (43,515)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	4,099	2,807
Depreciation and amortization	337	225
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(506)	(851)
Other assets	58	(6)
Accounts payable	(1,363)	(440)
Accrued contract research costs	2,580	1,150
Other accrued liabilities	(506)	(1,503)
Net cash used in operating activities	(41,144)	(42,133)
Cash flows from investing activities:
Purchases of marketable securities	(17,715)	(61,284)
Maturities of marketable securities	45,438	43,959
Purchases of property and equipment	(48)	(671)
Net cash provided by (used in) investing activities	27,675	(17,996)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of transaction costs, and exercise of common stock options	61,673	1,071
Proceeds from the sale of common stock to related parties, net of transaction costs	4,992
Proceeds from term loans		13,500
Payment of term loans	(4,854)	(2,165)
Payment of capital lease obligations	(21)	(6)
Net cash provided by financing activities	61,790	12,400
Net increase (decrease) in cash and cash equivalents	48,321	(47,729)
Cash and cash equivalents at beginning of period	48,490	81,512
Cash and cash equivalents at end of period	96,811	33,783
Supplemental disclosure of cash flow information:
Cash paid for interest	$ 1,055	$ 1,322