Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Operating activities:
Net loss	$ (61,577)	$ (21,384)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	78	50
Amortization of premium/discount on investments	287	271
Stock-based compensation	2,473	2,223
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,311)	(610)
Accounts payable	2,830	391
Accrued expense	6,362	(1,378)
Deferred revenue	40,000
Deferred rent	1,337	(27)
Net cash used in operating activities	(9,521)	(20,464)
Investing activities:
Purchase of equipment	(1,358)	(227)
Proceeds from the maturities of available for sale securities	64,874	34,790
Purchase of available for sale securities	(118,684)	(35,434)
Net cash used in investing activities	(55,168)	(871)
Financing activities:
Proceeds from the issuance of common stock, net of issuance costs	60,869	64,624
Proceeds from the sale of stock under employee stock purchase plan	105	109
Proceeds from the exercise of stock options	120	73
Payments on capital lease obligations	(6)	(3)
Net cash provided by financing activities	61,088	64,803
Decrease in cash and cash equivalents	(3,601)	43,468
Cash and cash equivalents at beginning of the period	49,778	32,780
Cash and cash equivalents at end of the period	46,177	76,248
Non-cash financing activities
Unpaid follow-on offering costs	$ 131
Assets acquired under capital lease		$ 12