Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities:
Net loss	$ (60,716)	$ (37,034)	$ (13,167)
Adjustments to reconcile net loss to net cash used in operating activities:
Gain on extinguishment of debt and other liabilities			(2,420)
Depreciation expense	96	49	1
Amortization of debt issuance costs			8
Amortization of premium/discount on investments	558	268	752
Stock-based compensation expense	4,714	6,010	1,564
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,001)	(686)	(217)
Other assets	(976)	(179)	(125)
Accounts payable	231	1,307	296
Accrued expenses	4,646	2,754	1,976
Other liabilities	41	28
Net cash used in operating activities	(52,407)	(27,483)	(11,332)
Investing activities:
Purchase of equipment	(414)	(229)	(19)
Proceeds from maturities of available for sale securities	63,901	12,585	1,990
Purchases of available for sale securities	(77,175)	(77,708)	(13,396)
Net cash used in investing activities	(13,688)	(65,352)	(11,425)
Financing activities:
Proceeds from the issuance of redeemable convertible preferred stock, net of issuance costs			40,047
Proceeds from the issuance of common stock, net of issuance costs	82,750	104,328	5
Proceeds from sale of stock under employee stock purchase plan	220
Proceeds from the exercise of stock options	130
Repurchase of treasury stock		(162)
Payments received on promissory notes issued in exchange for shares of common stock		237
Payments on capital lease obligations	(7)	(3)
Net cash provided by financing activities	83,093	104,400	42,331
Increase in cash and cash equivalents	16,998	11,565	19,574
Cash and cash equivalents at beginning of period	32,780	21,215	1,641
Cash and cash equivalents at end of period	49,778	32,780	21,215
Non-cash financing activities:
Conversion of series A, series B and series C preferred stock into common stock		244,727
Accretion of preferred stock to redemption value		$ 86,899	55,886
Unpaid initial public offering issuance costs			857
Unpaid follow-on offering costs	102
Assets acquired under capital lease	$ 12		12
Conversion of 2012 Series X preferred stock into Series C preferred stock			4,944
Series X Preferred Stock
Financing activities:
Proceeds from the issuance of 2012 Series X preferred stock, net of issuance costs			2,279
Non-cash financing activities:
Reclassification of 2012 Series X preferred stock from debt to preferred stock			$ 2,486