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	$ (83,948)	$ (32,811)	$ (31,154)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	22,759	13,489	3,254
Depreciation and amortization expense	112	96	77
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	332	(998)	502
Accounts payable	(1,309)	558	917
Accrued expense	16,783	(2,168)	4,087
Accrued interest, net of interest received on maturity of investments	3,647	(3,673)
Net cash used in operating activities	(41,624)	(25,507)	(22,317)
Cash flows from investing activities:
Purchases of marketable securities	(619,303)	(614,358)	(70,211)
Sales and maturities of marketable securities	650,182	234,304	48,330
Purchases of property and equipment, net of disposals	(172)	(22)	(125)
Net cash provided by (used in) investing activities	30,707	(380,076)	(22,006)
Cash flows from financing activities:
Proceeds from issuances of stock, excluding related parties, net of transaction costs		311,825	173,805
Proceeds from the sale of related party stock and exercise of common stock options, net of transaction costs	235	2,510
Net cash provided by financing activities	235	314,335	173,805
Net increase (decrease) in cash and cash equivalents	(10,682)	(91,248)	129,482
Cash and cash equivalents at beginning of period	57,379	148,627	19,145
Cash and cash equivalents at end of period	46,697	$ 57,379	148,627
Supplemental disclosure of cash flow information:
Obtaining a right-of-use asset in exchange for a lease liability	900
Purchases of property and equipment in accounts payable or accrued expenses at period end	$ 897		$ 250