Condensed Consolidated Statements of Cash Flows - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (41,802,000)	$ (20,252,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of preferred stock warrant liabilities	17,443,000	999,000
Stock-based compensation	1,912,000	197,000
Depreciation and amortization	71,000	3,000
Unrealized foreign exchange gain	(8,000)
Impairment on short-term investments	10,000
Deferred income taxes	(1,000)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(694,000)	(501,000)
Accrued liabilities	1,397,000	528,000
Accounts payable	483,000	597,000
Net cash used in operating activities	(21,189,000)	(18,429,000)
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from sale of short-term investments	10,031,000
Purchase of short-term investments	(21,000)	(10,003,000)
Change in restricted cash	(150,000)	(75,000)
Purchase of property and equipment	(381,000)	(39,000)
Net cash provided by (used in) investing activities	9,479,000	(10,117,000)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock upon initial public offering, net of issuance costs	147,270,000
Payment of deferred offering costs	(3,389,000)	(197,000)
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		57,545,000
Repayment of stock subscription receivable		1,159,000
Proceeds from exercise of redeemable convertible preferred stock warrants	1,507,000
Proceeds from exercise of common stock options	18,000	165,000
Proceeds from early exercise of stock options	13,000
Net cash provided by financing activities	139,876,000	58,672,000
Effect of exchange rate changes on cash and cash equivalents	(12,000)
NET INCREASE IN CASH AND CASH EQUIVALENTS	128,154,000	30,126,000
CASH AND CASH EQUIVALENTS - Beginning of period	29,154,000	2,429,000
CASH AND CASH EQUIVALENTS - End of period	157,308,000	32,555,000
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for income taxes	8,000
Cash paid for interest	0	0
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Change in redemption value of redeemable convertible preferred stock	(374,015,000)	(44,866,000)
Conversion of preferred stock to common stock	539,151,000
Issuance of redeemable convertible preferred stock on exercise of warrants	19,253,000
Deferred offering costs included in accounts payable and accrued liabilities		269,000
Property and equipment purchases included in accounts payable and accrued liabilities	80,000	6,000
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	(5,543,000)
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	$ 20,366,000
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		$ 433,000