Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Operating activities:
Net loss	$ (31,712)	$ (11,812)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	190	147
Amortization of intangible assets		83
Noncash interest expense	125	9
Amortization of premium on marketable securities	347	172
Stock based compensation expense	1,008	616
Gains on disposal of fixed assets		(146)
Change in fair value of preferred stock warrant liability		388
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(906)	(200)
Accounts payable	1,419	1,817
Accrued compensation	234	(430)
Deferred revenue		(5,630)
Accrued research and clinical liabilities	601	(289)
Other liabilities	(106)	(39)
Deferred rent	77	(34)
Net cash used in operating activities	(28,723)	(15,348)
Investing activities:
Proceeds from sale of property and equipment		170
Purchases of property and equipment	(335)
Purchase of marketable securities	(46,351)	(23,652)
Proceeds from maturities and sales of marketable securities	23,721	19,680
Net cash used in investing activities	(22,965)	(3,802)
Financing activities:
Payments of deferred offering costs	(29)	(1,510)
Proceeds from issuance of debt		4,840
Proceeds from issuances of Series E preferred stock, net		18,845
Proceeds from issuance of common stock	155	54
Proceeds from initial public offering, net of offering costs	63,912
Net cash provided by financing activities	64,038	22,229
Net increase in cash and cash equivalents	12,350	3,079
Cash and cash equivalents, beginning of period	10,947	3,338
Cash and cash equivalents, end of period	23,297	6,417
Supplemental cash flow disclosure:
Cash paid for interest	663
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
Issuance of preferred stock warrant issued with note payable		$ 79