Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (42,015)	$ (47,867)	$ (53,260)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	5,905	5,510	3,186
Depreciation	258	197	111
Non-cash restructuring charges		811
Change in fair value of preferred stock warrant liabilities			17,443
Other	31	15	34
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(115)	(362)	(1,087)
Accrued liabilities	990	(1,713)	5,302
Accounts payable	(1,217)	2,246	6
Net cash used in operating activities	(36,163)	(41,163)	(28,265)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(93)	(214)	(414)
Change in restricted cash	13	25	(150)
Proceeds from sale of property and equipment		43
Proceeds from sale of short-term investments			10,031
Purchase of short-term investments			(21)
Net cash (used in) provided by investing activities	(80)	(146)	9,446
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock upon follow-on offering, net of offering costs	27,422
Proceeds from exercise of common stock options	166	211	18
Payment of deferred offering costs		(15)	(3,389)
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
Net cash provided by financing activities	27,588	196	139,876
Effect of exchange rate changes on cash and cash equivalents	(4)	(60)	(31)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(8,659)	(41,173)	121,026
CASH AND CASH EQUIVALENTS-Beginning of period	109,007	150,180	29,154
CASH AND CASH EQUIVALENTS-End of period	100,348	109,007	150,180
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for income taxes	260	107	8
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Property and equipment purchases included in accounts payable and accrued liabilities	$ 4	$ 85	90
Change in redemption value of redeemable convertible preferred stock			(374,015)
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
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