Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash Flows From Operating Activities
Net loss	$ (32,644)	$ (21,712)	$ (12,377)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	404	361	281
Amortization of premiums on investments	446
Stock-based compensation	3,272	689	70
(Gain) Loss on disposal of property and equipment	(7)	2	5
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(673)	(1,544)	(282)
Other assets	(263)
Accounts payable	(131)	543	125
Accrued liabilities	(196)	2,058	310
Deferred rent, non-current	(141)	372	31
Net cash used in operating activities	(29,933)	(19,231)	(11,837)
Cash Flows From Investing Activities
Purchases of investments	(109,099)
Proceeds from maturity of investments	42,764
Purchase of property and equipment	(441)	(556)	(164)
Change in restricted cash		70	(9)
Net cash used in investing activities	(66,776)	(486)	(173)
Cash Flows From Financing Activities
Proceeds from issuance of common stock upon initial public offering, net		71,625
Net proceeds from issuance of convertible preferred stock		15,959	43,560
Proceeds from stock option exercises and employee stock plan purchases	845	282	65
Net cash provided by financing activities	845	87,866	43,625
Net (decrease) increase in cash and cash equivalents	(95,864)	68,149	31,615
Cash and cash equivalents at beginning of period	101,969	33,820	2,205
Cash and cash equivalents at end of period	6,105	101,969	$ 33,820
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Unpaid amounts related to property and equipment purchases	$ 26	110
Services settled through the issuance of common stock		$ 55