CONSOLIDATED 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	$ (192,256)	$ (99,854)	$ (50,284)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	20,410	9,688	4,866
Depreciation expense	830	317	150
Loss on disposal of property and equipment	0	0	10
Net accretion of discounts on marketable securities	(3,999)	0	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(10,321)	(5,770)	(637)
Operating lease assets	958
Accounts payable	10,862	3,991	2,904
Accrued expenses and other current liabilities	24,189	4,477	6,276
Operating lease liabilities	(319)
Other long-term liabilities	350	368	13
Net cash flows used in operating activities	(149,296)	(86,783)	(36,702)
Cash flows from investing activities:
Purchases of marketable securities	(817,142)	0	0
Maturities of marketable securities	261,216	0	0
Sales of marketable securities	100,780	0	0
Purchases of property and equipment	(4,935)	(1,125)	(406)
Increase in restricted investments	(441)	(1,069)	0
Net cash flows used in investing activities	(460,522)	(2,194)	(406)
Cash flows from financing activities:
Proceeds from public offerings, net of underwriting discounts and commissions	432,400	185,933	129,112
Proceeds from issuance of convertible preferred shares	0	0	52,300
Repayment of notes payable to related party	(1,294)	(187)	(187)
Payments of public offering costs	(620)	(674)	(4,395)
Payments of convertible preferred share issuance costs	0	0	(429)
Proceeds from exercise of stock options	5,888	915	0
Net cash flows provided by financing activities	436,374	185,987	176,401
Net increase (decrease) in cash and cash equivalents	(173,444)	97,010	139,293
Cash and cash equivalents at beginning of period	293,764	196,754	57,461
Cash and cash equivalents at end of period	120,320	293,764	196,754
Supplemental disclosure of cash flow information:
Cash paid for interest	67	84	95
Addition of operating lease asset included in accrued expenses and other current liabilities	562	0	0
Supplemental disclosure of non-cash investing and financing activities:
Amounts capitalized under build-to-suit lease transaction	0	11,885	0
Purchases of property and equipment included in accounts payable and accrued expenses and other current liabilities	661	0	78
Retirement of assets	217	0	0
Conversion of convertible preferred shares into common stock	0	0	244,538
Unsettled exercise of stock options	$ 553	$ 0	$ 0