Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Operating Activities:
Net loss	$ (57,415)	$ (47,199)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion of discount on convertible notes	2,650	2,241
Increase in lease liability	0	(180)
Depreciation expense	325	233
Stock-based compensation expense	10,012	10,780
Loss on disposal of property and equipment	0	312
Accretion of discount on investments	(835)	(484)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,583)	511
Accounts payable and accrued expenses	1,978	812
Operating lease liabilities	(62)
Net cash used in operating activities	(44,930)	(32,974)
Investing activities:
Cash acquired in connection with the Reverse Merger	0	76,348
Purchases of investments	(150,246)	(141,087)
Proceeds from maturities of investments	101,736	41,255
Proceeds from sale of property and equipment	0	20
Purchases of property and equipment	(14,015)	(570)
Payment of security deposit	0	(287)
Net cash used in investing activities	(62,525)	(24,321)
Financing activities:
Proceeds from issuance of common stock, net of issuance costs	86,080	78,518
Proceeds from exercise of options	31	149
Common stock repurchase	(727)	0
Net cash provided by financing activities	85,384	78,667
Net change in cash, cash equivalents and restricted cash	(22,071)	21,372
Cash, cash equivalents and restricted cash at beginning of period	112,791	18,349
Cash, cash equivalents and restricted cash at end of period	90,720	39,721
Supplemental disclosure of non-cash financing and investing activities:
Accrued purchases of property and equipment	3,535	0
Retirement of treasury stock	1,393	0
Net exercise of options	344	0
Unrealized gain (loss) on investments	244	(207)
Conversion of convertible preferred stock into common stock	0	41,466
Supplemental cash flow information:
Cash paid for interest	2,990	2,990
Cash paid for income taxes	$ 0	$ 2