Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Operating activities:
Net loss	$ (33,851)	$ (21,443)
Reconciliation of net loss to net cash used in operating activities:
Stock-based compensation	4,675	2,609
Depreciation and amortization	479	430
Noncash lease expense	133	109
Accretion of purchase discounts and amortization of premiums on investment securities, net	(293)	(689)
Other, net	(25)	2
Increase (decrease) in cash resulting from changes in:
Prepaid expenses and other current assets	(416)	24
Accounts payable and accrued expenses	1,560	(92)
Operating lease liability	(347)	(277)
Net cash used in operating activities	(28,085)	(19,327)
Investing activities:
Purchases of investment securities	(70,102)	(52,664)
Maturities of investment securities	104,115	87,170
Purchases of property and equipment	(72)	(464)
Net cash provided by investing activities	33,941	34,042
Financing activities:
Proceeds from issuance of stock in public offering, net	108,078
Proceeds from issuance of common stock, net	6,427
Proceeds from exercise of stock options	127	87
Repurchase of unvested shares		(59)
Net cash provided by financing activities	114,632	28
Net change in cash, cash equivalents and restricted cash	120,488	14,743
Cash, cash equivalents and restricted cash at beginning of period	40,826	45,473
Cash, cash equivalents and restricted cash at end of period	161,314	60,216
Noncash investing and financing activities:
Issuance of shares under Stock Purchase Plan	407	379
Amounts accrued for purchases of property and equipment	97
Change in accrued public offering costs	222
Change in unvested stock liability	$ 15	$ 71