Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Operating activities:
Net loss	$ (20,222)	$ (20,383)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	230	85
Noncash interest expense	101	76
Amortization of premium on marketable securities	217	218
Stock based compensation expense	1,018	554
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(351)	(1,044)
Accounts payable	(540)	1,402
Accrued compensation	(81)	(146)
Accrued research and clinical liabilities	(1,101)	(1,069)
Other liabilities	49	(68)
Deferred rent	(3)	(29)
Net cash used in operating activities	(20,683)	(20,404)
Investing activities:
Purchase of marketable securities	(49,902)	(39,731)
Proceeds from maturities of marketable securities	22,460	4,705
Purchases of property and equipment	(287)	(23)
Changes in restricted cash	(193)
Net cash used in investing activities	(27,922)	(35,049)
Financing activities:
Proceeds from initial public offering, net of offering costs		63,912
Proceeds from issuances of debt	5,000
Proceeds from issuance of common stock	59	68
Principal payments under notes payable	(1,621)
Net cash provided by financing activities	3,438	63,980
Net (decrease) increase in cash and cash equivalents	(45,167)	8,527
Cash and cash equivalents, beginning of period	54,220	10,947
Cash and cash equivalents, end of period	9,053	19,474
Supplemental cash flow disclosure:
Cash paid for interest	425	453
Supplemental disclosure of noncash financing activities:
Conversion of preferred stock to common stock and additional paid-in capital		102,023
Reclassification of preferred stock warrants liability to additional paid-in capital		157
Issuance of common stock warrants in connection with a loan amendment		$ 130