Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Operating activities:
Net loss	$ (12,637)	$ (29,401)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion on marketable securities	150	(625)
Depreciation and amortization expense	68	55
Stock-based compensation expense	1,734	1,607
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,080)	(1,786)
Accounts payable	(4,251)	5,197
Accrued liabilities	(4,386)	(1,263)
Other liabilities	19	(8)
Net cash used in operating activities	(20,383)	(26,224)
Investing activities:
Purchases of property and equipment		(48)
Maturities of marketable securities	57,500	67,500
Purchases of marketable securities		(77,104)
Net cash provided by (used in) investing activities	57,500	(9,652)
Financing activities:
Proceeds from public offering, net of issuance costs		49,748
Proceeds from at-the-market offering, net of issuance costs		627
Taxes paid related to net share settlement of restricted stock units	(2)	(9)
Net cash (used in) provided by financing activities	(2)	50,366
Net increase in cash, cash equivalents and restricted cash	37,115	14,490
Cash, cash equivalents and restricted cash at beginning of period	34,019	6,881
Cash, cash equivalents and restricted cash at end of period	71,134	21,371
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable	$ 2	11
Issuance costs associated with at-the-market offering included in accounts payable		$ 45