Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Operating activities:
Net loss	$ (21,384)	$ (17,315)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	50	20
Amortization of premium/discount on investments	271	25
Stock-based compensation expense	2,223	3,889
Changes in operating assets and liabilities:
Accounts receivable		21
Prepaid expenses and other current assets	(610)	(950)
Accounts payable and accrued expenses	(987)	1,603
Other liabilities	(27)	30
Net cash used in operating activities	(20,464)	(12,677)
Investing activities:
Purchase of equipment	(227)	(171)
Proceeds from maturities of available for sale securities	34,790	6,745
Purchases of available for sale securities	(35,434)	(24,291)
Net cash used in investing activities	(871)	(17,717)
Financing activities:
Proceeds from the issuance of common stock, net of issuance costs	64,624	104,596
Proceeds from employee stock purchase plan	109
Proceeds from the exercise of stock options	73
Repurchase of treasury stock		(21)
Payments on capital lease obligations	(3)	(2)
Net cash provided by financing activities	64,803	104,573
Increase in cash and cash equivalents	43,468	74,179
Cash and cash equivalents at beginning of period	32,780	21,215
Cash and cash equivalents at end of period	76,248	95,394
Non-cash financing activities:
Accretion of preferred stock to redemption value		86,900
Unpaid initial public offering issuance costs		121
Assets acquired under capital lease	$ 12	$ 12