Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (25,508)	$ (17,385)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	961	486
Stock-based compensation	3,000	1,544
Amortization of premiums and discounts on marketable securities	(896)	(192)
Loss on disposal of property and equipment		97
Changes in operating assets and liabilities
Prepaid expenses, other current assets and other long-term assets	(1,187)	(1,272)
Accounts payable, accrued liabilities and other current liabilities	1,534	1,619
Deferred rent		(216)
Operating lease liabilities	(571)
Net cash used in operating activities	(22,667)	(15,319)
Cash flows from investing activities:
Purchases of property and equipment	(589)	(1,003)
Purchases of marketable securities	(86,571)	(72,995)
Maturities of marketable securities	105,901
Proceeds from sale of property and equipment		10
Net cash provided by (used in) investing activities	18,741	(73,988)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs		77,695
Proceeds from issuance of common stock under employee equity plans	192	213
Net cash provided by financing activities	192	77,908
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(83)	(53)
Net decrease in cash, cash equivalents and restricted cash	(3,817)	(11,452)
Cash, cash equivalents and restricted cash at the beginning of period	24,182	51,046
Cash, cash equivalents and restricted cash at the end of period	20,365	39,594
Supplemental disclosures of noncash investing and financing information:
Reclassification of employee stock liability to equity upon vesting	37	29
Addition of tenant improvement paid by landlord		2,703
Purchase of property and equipment in accounts payable	$ 17	7
Conversion of redeemable convertible preferred stock into common stock		$ 77,931