Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (47,867)	$ (53,260)	$ (23,873)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of preferred stock warrant liabilities		17,443	1,380
Stock-based compensation	5,510	3,186	302
Depreciation and amortization	197	111	12
Non-cash restructuring charges	811
Other	15	34	(2)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(362)	(1,087)	(531)
Accrued liabilities	(1,713)	5,302	740
Accounts payable	2,246	6	186
Net cash used in operating activities	(41,163)	(28,265)	(21,786)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(214)	(414)	(187)
Change in restricted cash	25	(150)	(75)
Proceeds from sale of property and equipment	43
Proceeds from sale of short-term investments		10,031
Purchase of short-term investments		(21)	(10,020)
Net cash (used in) provided by investing activities	(146)	9,446	(10,282)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from exercise of common stock options	211	18	305
Payment of deferred offering costs	(15)	(3,389)	(316)
Proceeds from issuance of common stock upon initial public offering, net of issuance costs		147,270
Proceeds from exercise of redeemable convertible preferred stock warrants		1,507
Proceeds from early exercise of stock options		13	100
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs			57,545
Repayment of stock subscription receivable			1,159
Net cash provided by financing activities	196	139,876	58,793
Effect of exchange rate changes on cash and cash equivalents	(60)	(31)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(41,173)	121,026	26,725
CASH AND CASH EQUIVALENTS-Beginning of period	150,180	29,154	2,429
CASH AND CASH EQUIVALENTS-End of period	109,007	150,180	29,154
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for income taxes	107	8
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Change in redemption value of redeemable convertible preferred stock		(374,015)	(49,849)
Conversion of preferred stock to common stock		539,151
Issuance of redeemable convertible preferred stock on exercise of warrants		19,253
Deferred offering/financing costs included in accounts payable and accrued liabilities		154	233
Property and equipment purchases included in accounts payable and accrued liabilities	$ 85	90	39
Series B and B-1 Redeemable Convertible Preferred Stock [Member]
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment of Series B and B-1 redeemable convertible preferred stock cumulative dividend		(5,543)
Series C and D Redeemable Convertible Preferred Stock [Member]
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Fair value of common stock issued in settlement of Series C and Series D redeemable convertible preferred stock cumulative dividends		$ 20,366
Series C Redeemable Convertible Preferred Stock [Member]
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Conversion of long-term note payable and accrued interest into Series C redeemable convertible preferred stock			$ 433