STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities:
Net loss	$ (176,813)	$ (102,808)	$ (41,290)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	757	618	335
Amortization of operating lease right-of-use assets	964
Accretion (amortization) of premiums and discounts on investments	(3,698)	(1,094)	(42)
Amortization of Term Loan discount and issuance costs	2,173	1,316	0
Stock-based compensation	25,168	5,152	876
Changes in fair value of compound derivative liability and warrants	816	(188)	0
Changes in fair value of preferred stock tranche obligation	0	0	(5,649)
Other non-cash items	(92)	0	0
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,492)	(1,338)	(1,157)
Accounts payable	(2,621)	4,573	1,757
Accrued expenses and other liabilities	25,955	(1,087)	4,769
Operating lease liabilities	(707)
Net cash used in operating activities	(129,590)	(94,856)	(40,401)
Investing activities:
Purchase of investments	(497,492)	(233,928)	(76,846)
Maturities of investments	371,417	86,429	39,903
Purchase of property and equipment	(1,423)	(855)	(1,004)
Net cash used in investing activities	(127,498)	(148,354)	(37,947)
Financing activities:
Proceeds from equity offerings, net	217,930	237,750	0
Payments of offering costs	(921)	(6,564)	0
Proceeds from exercise of common stock under equity incentive plans	3,036	632	14
Proceeds from issuance of convertible preferred stock, net	0	85	82,465
Proceeds from leasehold improvement loan	0	276	0
Repayments of leasehold improvement loan	106	113	39
Proceeds (payments) under Term Loan, net	18,551	38,542	0
Net cash provided by financing activities	238,490	270,608	82,440
Net increase (decrease) in cash and cash equivalents	(18,598)	27,398	4,092
Cash and cash equivalents at beginning of year	37,172	9,774	5,682
Cash and cash equivalents at end of year	18,574	37,172	9,774
Supplemental disclosures
Cash paid for interest	3,348	1,612	0
Supplemental disclosures of non-cash investing and financing activities
Right-of-use assets obtained in exchange for lease obligations	8,084
Warrants and compound derivative liability related to Term Loan	535	1,693	0
Purchase of property and equipment in accounts payable and accrued expenses	820	27	0
Series C Preferred Stock
Supplemental disclosures of non-cash investing and financing activities
Series C fair value of preferred stock obligation upon closing	$ 0	$ 0	$ 2,278