STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net loss	$ (85,372)	$ (43,538)	$ (23,552)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	15,764	4,570	402
Net amortization of premiums and discounts on marketable securities	(2,664)	(1,310)	0
Amortization of beneficial conversion feature related to convertible promissory notes payable to related parties	0	0	853
Benefit from deferred income taxes	0	0	(291)
Depreciation and amortization	1,508	242	241
Noncash lease expense	275	0	0
Noncash interest related to convertible promissory notes payable to related parties	0	0	228
Loss on extinguishment of debt facility	0	0	159
Noncash interest related to debt facility	0	0	101
Accretion of tenant improvement allowance	0	(82)	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	463	(1,489)	(637)
Other long-term assets	(564)	313	(150)
Accounts payable	3,571	76	510
Accrued expenses and other current liabilities	4,077	2,063	(432)
Other long-term liabilities	(70)	705	0
Net cash used in operating activities	(63,012)	(38,450)	(22,568)
Cash flows from investing activities
Purchases of marketable securities	(541,701)	(236,601)	0
Proceeds from maturities of marketable securities	230,500	92,500	0
Purchases of property and equipment	(770)	(6,946)	(264)
Net cash used in investing activities	(311,971)	(151,047)	(264)
Cash flows from financing activities
Proceeds from issuance of common stock, net of issuance costs	377,525	138,357	0
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	0	92,331
Proceeds from issuance of convertible promissory notes, net of issuance costs	0	0	7,414
Repayment of debt facility	0	0	(5,250)
Proceeds from exercise of stock options, net of repurchases	2,448	345	228
Proceeds from issuance of common stock under 2018 ESPP	1,190	0	0
Payments for deferred financing costs	0	0	(100)
Proceeds from repayment of recourse promissory note	0	50	0
Net cash provided by financing activities	381,163	138,752	94,623
Net increase (decrease) in cash, cash equivalents and restricted cash	6,180	(50,745)	71,791
Cash, cash equivalents and restricted cash, beginning of period	34,462	85,207	13,416
Cash, cash equivalents and restricted cash, end of period	40,642	34,462	85,207
Supplemental disclosures
Cash paid for interest	0	0	228
Noncash investing and financing items:
Right-of-use assets obtained in exchange for lease obligations	6,050	0	0
Conversion of convertible promissory notes payable to related parties	0	0	7,642
Recognition of beneficial conversion feature related to convertible promissory notes to related parties, net of benefit for income taxes	0	0	1,867
Lessor funded lease incentives included in property and equipment	0	1,386	0
Reclassification of beneficial conversion feature related to convertible promissory notes payable to related parties, net of tax expense	0	0	1,305
Property and equipment purchased in accounts payable	0	313	89
Deferred initial public offering costs in accounts payable	0	0	63
Vesting of restricted common stock subject to repurchase	$ 20	$ 24	$ 28