Condensed Consolidated Statements of Cash Flows - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating activities:
Net loss	$ (40,859,000)	$ (26,664,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	85,000	33,000
Amortization of premium/discount on investments	418,000	124,000
Stock-based compensation expense	3,386,000	5,125,000
Changes in operating assets and liabilities:
Accounts receivable	34,000	78,000
Prepaid expenses and other current assets	190,000	(688,000)
Accounts payable and accrued expenses	7,276,000	3,683,000
Other liabilities	(25,000)	29,000
Net cash used in operating activities	(29,495,000)	(18,280,000)
Investing activities:
Purchase of equipment	(342,000)	(208,000)
Proceeds from maturities of available for sale securities	49,547,000	6,990,000
Purchases of available for sale securities	(64,531,000)	(64,497,000)
Net cash used in investing activities	(15,326,000)	(57,715,000)
Financing activities:
Proceeds from the issuance of common stock, net of issuance costs	78,579,000	104,293,000
Proceeds from employee stock purchase plan	109,000
Proceeds from the exercise of stock options	70,000
Repurchase of treasury stock		(79,000)
Payments received on promissory notes issued in exchange for shares of common stock		237,000
Payments on capital lease obligations	(5,000)	(3,000)
Net cash provided by financing activities	78,753,000	104,448,000
Increase in cash and cash equivalents	33,932,000	28,453,000
Cash and cash equivalents at beginning of period	32,780,000	21,215,000
Cash and cash equivalents at end of period	66,712,000	49,668,000
Non-cash financing activities:
Accretion of preferred stock to redemption value		86,899,000
Unpaid initial public offering issuance costs		15,000,000
Assets acquired under capital lease	$ 12,000	$ 12,000