Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (61,342)	$ (39,981)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	564	415
Stock-based compensation expense	5,058	2,516
Non-cash interest expense	306	45
Amortization and accretion on marketable securities	(645)
Change in fair value of preferred stock warrant liability		110
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(321)	(2,219)
Operating lease, right-of-use assets	2,384
Accounts payable	(623)	1,481
Accrued expenses and other current liabilities	1,651	2,399
Operating lease liabilities	(2,322)
Deferred rent		(87)
Other assets		18
Net cash used in operating activities	(55,290)	(35,303)
Cash flows from investing activities:
Purchase of marketable securities	(65,118)
Purchases of property and equipment	(683)	(124)
Proceeds from maturities and sales of marketable securities	55,675
Net cash used in investing activities	(10,126)	(124)
Cash flows from financing activities:
Proceeds from initial public offering, net of underwriting discounts and commissions		55,801
Proceeds from issuance of convertible preferred stock, net of issuance costs		99,606
Proceeds from issuance of long-term debt	29,649
Payment of debt issuance costs	(125)
Payments on long-term debt, including final payment		(4,698)
Proceeds from repayment of promissory notes issued upon early exercise of stock options		290
Payments of initial public offering costs		(3,524)
Proceeds from issuance of common stock upon stock option exercises	132	127
Proceeds from exercises of common stock warrants		79
Net cash provided by financing activities	29,801	147,681
Net increase (decrease) in cash, cash equivalents and restricted cash	(35,615)	112,254
Cash, cash equivalents and restricted cash at beginning of period	115,017	16,646
Cash, cash equivalents and restricted cash at end of period	79,402	128,900
Supplemental disclosure of cash flow information:
Interest paid	784	105
Supplemental disclosure of noncash investing and financing information:
Vesting of common stock subject to repurchase		4
Deferred offering costs included in accounts payable and accrued expenses	61	$ 114
Jefferies Sales Agreement [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock from the Jefferies sales agreement (net of issuance costs, $0.2 million)	145
Payments of initial public offering costs	$ (200)