Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities:
Net loss	$ (27,411)	$ (15,037)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	815	741
Stock-based compensation expense	1,781	246
Gain on sale of equipment		(15)
Change in fair value of redeemable convertible preferred stock call option liability, net		(314)
Change in operating assets and liabilities:
Prepaid expenses and other assets	520	(192)
Other long-term assets		38
Accounts payable	(372)	413
Accrued liabilities	1,027	1,089
Other long-term liabilities	(75)	503
Deferred revenue	(10,649)	(10,649)
Net cash used in operating activities	(34,364)	(23,177)
Cash flows from investing activities:
Purchases of property and equipment	(854)	(1,374)
Proceeds from sale of equipment		134
Changes in restricted cash		60
Net cash used in investing activities	(854)	(1,180)
Cash flows from financing activities:
Proceeds from issuance of common stock	226	352
Payments of deferred offering costs	(153)	(935)
Net cash provided by financing activities	73	45,254
Net (decrease) increase in cash and cash equivalents	(35,145)	20,897
Cash and cash equivalents, beginning of period	112,265	43,648
Cash and cash equivalents, end of period	77,120	64,545
Supplemental disclosures of noncash investing and financing information
Unpaid deferred offering costs	461	778
Vesting of early exercised options and restricted stock	226	120
Unpaid portion of property and equipment purchases included in period-end accounts Payable	$ 7	48
Accretion of dividends on redeemable convertible preferred stock		4,530
Accretion of redeemable convertible preferred stock to redemption value		88
Series B Redeemable Convertible Preferred Stock
Cash flows from financing activities:
Proceeds from issuance of preferred stock, net		$ 45,837