Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Operating activities:
Net loss	$ (10,408)	$ (8,574)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	101	42
Noncash interest expense	45	38
Amortization of premium on marketable securities	59	54
Stock based compensation expense	491	181
Changes in operating assets and liabilities:
Contract receivables	40	53
Prepaid expenses and other assets	(138)	(1,025)
Accounts payable	(1,563)	3,329
Accrued compensation	(454)	200
Accrued research and clinical liabilities	382	(2,153)
Other liabilities	155	73
Deferred rent	(8)	(19)
Net cash used in operating activities	(11,298)	(7,801)
Investing activities:
Purchases of property and equipment	(35)	(5)
Purchase of marketable securities	(31,005)
Proceeds from maturities of marketable securities	22,460	2,395
Changes in restricted cash	(193)
Net cash provided by (used in) investing activities	(8,773)	2,390
Financing activities:
Proceeds from issuance of common stock in initial public offering, net of underwriting costs		64,413
Proceeds from the exercise of stock options	59
Principal payments under notes payable	(811)
Net cash provided by (used in) financing activities	(752)	64,413
Net increase (decreases) in cash and cash equivalents	(20,823)	59,002
Cash and cash equivalents, beginning of period	54,220	10,947
Cash and cash equivalents, end of period	33,397	69,949
Supplemental cash flow disclosure:
Cash paid for interest	219	225
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