Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities:
Net loss	$ (85,158)	$ (34,779)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	10,535	13,777
Depreciation and amortization expense	240	56
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,072)	(906)
Accounts payable	3,826	(2,041)
Accrued expense	16,311	5,607
Accrued interest, net of interest received on maturity of investments	554	268
Net cash used in operating activities	(54,764)	(18,018)
Cash flows from investing activities:
Purchases of marketable securities	(153,869)	(266,116)
Sales and maturities of marketable securities	224,015	299,103
Purchases of property and equipment, net of disposals	(321)	(40)
Net cash provided by investing activities	69,825	32,947
Cash flows from financing activities:
Proceeds from the sale of related party stock and exercise of common stock options, net of transaction costs	109	200
Net cash provided by financing activities	109	200
Net increase in cash and cash equivalents	15,170	15,129
Cash and cash equivalents at beginning of period	46,697	57,379
Cash and cash equivalents at end of period	61,867	72,508
Supplemental disclosure of cash flow information:
Obtaining a right-of-use asset in exchange for a lease liability	$ 0	$ 900