Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (38,619)	$ (41,802)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	3,965	1,912
Change in fair value of preferred stock warrant liabilities		17,443
Depreciation and amortization	151	71
Non-cash restructuring charges (note 12)	811
Other	36	1
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(843)	(694)
Accrued liabilities	7,184	1,397
Accounts payable	(184)	483
Net cash used in operating activities	(27,499)	(21,189)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(196)	(381)
Change in restricted cash	25	(150)
Proceeds from sale of property and equipment	43
Purchase of short-term investments		(21)
Proceeds from sale of short-term investments		10,031
Net cash (used in) provided by investing activities	(128)	9,479
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from exercise of common stock options	194	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 early exercise of stock options		13
Proceeds from exercise of redeemable convertible preferred stock warrants		1,507
Net cash provided by financing activities	179	139,876
Effect of foreign exchange rate changes on cash and cash equivalents	(50)	(12)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(27,498)	128,154
CASH AND CASH EQUIVALENTS - Beginning of period	150,180	29,154
CASH AND CASH EQUIVALENTS - End of period	122,682	157,308
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for income taxes	94	8
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
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
Property and equipment purchases included in accounts payable and accrued liabilities	$ 10	80
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