Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Operating Activities:
Net loss	$ (38,131)	$ (31,110)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion of discount on convertible notes	1,733	1,454
Increase in lease liability	0	(109)
Depreciation expense	204	157
Stock-based compensation expense	7,319	8,168
Loss on disposal of property and equipment	0	205
Accretion of discount on investments	(503)	0
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(2,398)	(651)
Accounts payable and accrued expenses	(2,265)	665
Operating lease liabilities	(40)
Net cash used in operating activities	(34,081)	(21,339)
Investing activities:
Cash acquired in connection with the Reverse Merger	0	76,348
Purchases of investments	(140,336)	(118,819)
Proceeds from maturities of investments	49,936	21,232
Proceeds from sale of property and equipment	0	20
Purchases of property and equipment	(7,305)	(117)
Net cash used in investing activities	(97,705)	(21,336)
Financing activities:
Proceeds from issuance of common stock, net of issuance costs	86,080	78,518
Common stock repurchase	(727)	0
Net cash provided by financing activities	85,353	78,518
Net change in cash, cash equivalents and restricted cash	(46,433)	35,843
Cash, cash equivalents and restricted cash at beginning of period	112,791	18,349
Cash, cash equivalents and restricted cash at end of period	66,358	54,192
Supplemental disclosure of non-cash financing and investing activities:
Accrued purchases of property and equipment	716	0
Retirement of treasury stock	1,395	0
Net exercise of options	344	0
Unrealized gain (loss) on investments	271	(79)
Conversion of convertible preferred stock into common stock	0	41,466
Supplemental cash flow information:
Cash paid for interest	$ 1,495	$ 1,495