Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (22,906)	$ (14,489)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	599	426
Stock-based compensation expense	3,409	1,762
Amortization of premium and accrued interest on marketable securities	(179)	(129)
Other	(48)	40
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	6,345	(91)
Other assets	(8)	(18)
Operating lease right-of-use asset	557	539
Accounts payable	2,514	(136)
Accrued expenses and other current liabilities	(3,429)	(2,363)
Lease liability	(470)	1,242
Net cash used in operating activities	(13,616)	(13,217)
Cash flows from investing activities:
Purchases of marketable securities	(127,733)	(22,454)
Maturities of marketable securities	61,200	27,385
Purchases of property and equipment	(4,210)	(2,186)
Net cash (used in) provided by investing activities	(70,743)	2,745
Cash flows from financing activities:
Proceeds from offerings of common stock, net of issuance costs	140,923	0
Proceeds from issuance of common stock pursuant to option exercises	9,650	162
Taxes paid related to net share settlement of restricted stock units	(1,922)	(504)
Repayment of loan	(27)	(28)
Net cash provided by (used in) financing activities	148,624	(370)
Net increase (decrease) in cash and cash equivalents and restricted cash	64,265	(10,842)
Cash and cash equivalents and restricted cash at beginning of period	66,896	155,948
Cash and cash equivalents and restricted cash at end of period	131,161	145,106
Supplemental schedule of noncash investing and financing information
Fixed assets in accounts payable, accrued expenses and other current liabilities	$ 3,474	$ 413