Consolidated 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	$ (53,260)	$ (23,873)	$ (6,832)
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	202
Stock-based compensation	3,186	302	22
Depreciation and amortization	111	12	1
Unrealized foreign exchange loss	27
Noncash interest expense on short-term promissory notes and long-term note payable			2,760
Common stock issuance for milestone payment			57
Other	7	(2)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,087)	(531)	(10)
Accrued liabilities	5,302	740	356
Accounts payable	6	186	(89)
Accrued interest			(39)
Net cash used in operating activities	(28,265)	(21,786)	(3,572)
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from sale of short-term investments	10,031
Purchase of short-term investments	(21)	(10,020)
Change in restricted cash	(150)	(75)
Purchase of property and equipment	(414)	(187)	(2)
Net cash provided by (used in) investing activities	9,446	(10,282)	(2)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock upon initial public offering, net of issuance costs	147,270
Payment of deferred offering costs	(3,389)	(316)
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		57,545	2,219
Repayment of stock subscription receivable		1,159
Proceeds from exercise of redeemable convertible preferred stock warrants	1,507
Proceeds from short-term convertible promissory notes			2,445
Proceeds from exercise of common stock options	18	305	25
Proceeds from early exercise of stock options	13	100
Net cash provided by financing activities	139,876	58,793	4,689
Effect of exchange rate changes on cash and cash equivalents	(31)
NET INCREASE IN CASH AND CASH EQUIVALENTS	121,026	26,725	1,115
CASH AND CASH EQUIVALENTS-Beginning of period	29,154	2,429	1,314
CASH AND CASH EQUIVALENTS-End of period	150,180	29,154	2,429
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for income taxes	8
Cash paid for interest	0	0	0
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Change in redemption value of redeemable convertible preferred stock	(374,015)	(49,849)	(5,713)
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	90	39
Conversion of short-term convertible promissory notes and accrued interest into Series C redeemable convertible preferred stock			5,383
Subscription receivable from investors on issuance of Series C redeemable convertible preferred stock			1,159
Issuance of preferred stock warrants with redeemable convertible preferred stock			57
Settlement of premium conversion derivative in redeemable convertible preferred stock			$ 2,415
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